Tons of info on this and other sites, readily available parts, and only minimal tools REQUIRED to make the most common mods. Call me a heretic, but I've seen some of Rich's work, and dealt with him personally, neither was a pleasant experience. 'Nuff said on that subject, obviously, your mileage DOES vary
As to other builders, the Crosmans seem to get all the attention, but I know at least a few of them do QB's, Mountain Air comes to mind first. And Richard's Microfit rifle stocks has ONE air gun in their catalog, and that's the QB, so if you're looking for an inexpensive custom stock, that would be a good place to start.
But for the basic build, I'll say it again, do it yourself. It's incredibly satisfying taking one of these basic rifles and making it yours. Warning, though, it's seriously addictive, I've recently "finished" my first, and have plans for at least 2 more. Later.