DH and I had planned to go through an agency for a closed, domestic adoption but then discovered we were pregnant. (Surprise, lol!) We still plan to adopt our second child, but we'll be going through the Texas DFPS to adopt an older infant or toddler from the foster care system. The cost is minimal and much easier for us to handle than the original 15-20K we'd been quoted through private agencies.
Keep in mind that international adoptions are going to be just as expensive, if not more. You'll have to travel to the country once or twice, usually for extended periods. You'll also have to pay for all kinds of document translations, jump through all the State Department hoops, etc. We were quoted 18K for China and as high as 35K for a handful of other countries. So it's big bucks...
Adoption.com is a good place to start. You might find more agencies through google. Just be careful when you make your final choice. Don't choose an agency that requires all the money up front or won't refund if you choose to stop using the agency.
Also, try the
forums at Adoption.com. You'll likely find someone in your area you can question or get some recommendations.
Good luck!