My son is 9 now, but still functioning on a pre-school level. We tried to teach him sign language for years, and, finally, he is using it and I am understanding it. (It was really hard for me, when I first started.) My son has severe apraxia but also is severely behind in all aspects of language (and everything else, for that matter).
It is very doubtful that his pronunciation will ever get good enough for strangers to understand him. I still don't understand most of what he says, unless he knows how to sign it or is repeating after someone. Sign has been very helpful for us. I'm so glad we stuck with it, even though it took years for him to begin initiating signs.
Over the years, we have collected a good number of resources. I'll try to give you a few ideas. We use Signing Exact English (SEE) because it is not another language but just another way to express our language (just like writing uses exact English to "speak" our language). I have no interest, whatsoever, in American Sign Language (ASL), because I have no reason to expect my son to spend his life around deaf people. I want him to learn English, my native language and ASL qualifies as a separate language, with its own grammar.
That said, we find that we must use many ASL resources (because that's most of what is out there) and that has created no problems. Since most of our ASL resources are videos designed to teach children words, not sentences, they have only been teaching him English, not ASL grammar. Yes, some SEE signs are different from the same word in ASL. Most, it seems, are the same. But the videos have been so useful because he loves to watch them. He often comes up with signs I did not even know that he knew. He learned it from a video.
We own the large SEE dictionary. It cost about $40, but I wouldn't part with it. A few years later, I splurged and bought the SEE dictionary on CD Rom, for about $200. (Both are from
http://www.modernsignspress.com.) We have really used it a lot. I search for words (or even sentences) and print a list of instructions. I've cut them out and "clear-contact-papered" them on the walls, near the words they represent. This helps me to remember the signs. We have an especially lot of words on the bathroom wall. We have on and off by the light fixtures. We have a few on the gate to the kitchen. We have some by several windows, so we can talk about what we see. We have more by the closet, showing the words we need to get dressed to go out. More can go on the dresser (for cloths), on the refrigerator (if it is not in an off-limit area, like ours), or wherever.
I also print out the words I need for reading simple books. I glue them on the back cover, as a reference. (By the way, I also rewrote some simple board books, to make them simpler. I just pasted new words over the old. Now he likes me to read them because he understands some of what I'm reading.)
Anyway, you may not want to start out with such expensive resources, unless you expect signing to be a permanent way for your son to communicate. You should get some videos, though. The Bee Smart Baby series has sign language (on their DVD versions only) on some of their titles (www.BabyBumbleBee.com). You have to put the DVD in the right mode. It does the usual thing it does on their videos: they say and show a noun several times. But, at the end, they sign all of the words for you. I wish they had mixed it all in, but they are still nicely done, but maybe a little young for my son. He has a big appetite for "boring" dictionary type sign language tapes (and language CD ROMs, too). I guess he really does want to learn language; it is just very hard for him.
We both loved "Oh See...Can You Say?” I liked it because they help you understand how the sign is related to the word they are signing. For instance: they peel a banana and then peel a finger (the sign for banana). I highly recommend this one. He also loves Talking Hands, some of the Say, Sing and Sign series. (He learned the alphabet from their ABC video and colors from their Colors one. The numbers one is nice, too and the Animals one is ok, too. We don't have Nursery Rhymes or any of the others, if there are others.) He also likes Baby See 'n Sign.
There is a lot of repetition among the videos, of course, since they are all beginner videos, and, since there is more than one sign for many words, they sometimes disagree on how to sign a word.
My son will even watch Beginning Reading and Sign Language, a very amateur production where they seem to sign anything that it is easy to illustrate, without regard to whether it is a word you would use often. He absolutely loved (until he broke it) S.E.E. Me Sing, a really cute video with a "story" that makes absolutely no sense. (He doesn't understand stories, yet, anyway.) That one was from Modern Signs Press.
We may have a few others around here (probably in the broken and waiting to fix stack); I don't remember. Anyway, most of the videos I mentioned are available from www.amazon.com and, I think, the only other resource you need for anything I recommended is the Modern Signs Press link I gave above. I managed to find absolutely nothing locally, to purchase. The library had a few things, but the only one useful for a younger child was "Oh See...Can You Say?”
I've found a new series (3 volumes) of videos (or DVDs)I plan to buy, Signing Time, My First Signs. The first volume is available from Amazon and got really good review. My son knows some of the signs, but not all of them and, since he loves music, I think he will love these. They are supposed to have nice music and they also sell an audio CD with all the songs on it. Little Signers (
http://www.littlesigners.com/) will give you a 15% discount if you buy all 3 volumes. They also recommend Sign Songs and we may purchase that also. And they sell a sign language software program for children that I may try, too.
Good luck.