I have an unusual ethnicity requirement and would have loved to have been able to find a clinic donor to cycle with, but there were none available so I turned to agencies. I registered with MANY agencies and the donor search goes so much faster this way as many of the databases are huge.
For starters, here is a list of agencies that follow the ASRM guidelines. Most have the agency website listed and from there you can usually register and get a password to view the donor databases right away.
Most of them have all their fees and prices stated upfront on the website. Some don't, I have found them to be very willing to provide their price list if you ask them.
From my experience, there are some pretty big downsides to using an agency, aside from the huge extra costs:
1. You would sign an agency contract as well as a donor contract. The agency contracts I have seen are terrible. Most like you to sign soon after choosing a donor. If the donor backs out for any reason, or fails her medical screening, you could lose 1/2 to the entire agency fee, as it is non-refundable. Now I have already had several donors back out and even one proven donor who failed medical screening, so this was a big deal to me. The agency will say they will use the fee to find you another donor, but for me with my unusual ethnicity requirement the chances of the same agency finding another donor were slim, and I wouldn't want to wait months to a year for them anwyay. I would always negotiate the agency contract terms first, some agencies were more willing than others. If you aren't confident in negotiating contract terms, you could have an ART attorney do it for you. I did my own negotiations and found this to be a huge hassle, but then again I would have lost 1000s of $$ if I hadn't done it. Most people don't have donors who back out and everything goes just fine when they sign the contract as-is, but I ran into some particularly bad luck with several donors in a row not even making it past the screening step so I was very cautious.
2. Agency donors aren't medically screened yet (unless they have donated before), whereas the clinic donors are often pre-screened.
3. The outside agency is another party through which information has to flow, allowing more chances of miscommunication. In my case, the donor told the agency something very important, but the agency did not pass that information to the clinic coordinator when they were supposed to. The clinic finally found out this information quite late and it could have been impacted the cycle in a very negative way, but they did scramble at the last minute and worked it out. If I were using a clinic donor, this type of hiccup wouldn't have happened.
Despite the drawbacks, I was able to find a match for my ethnic mix through an agency fairly quickly, and as an added bonus, the donor happens to be local to the clinic too. We plan to cycle this month.