The NPIP testing is a government program, so I would be wary of anyone other than those involved with your county agents doing the testing. The cost shouldn't be high as they expect you to test at least 10% of your flock but I have also heard of some states requiring 100% of the birds be tested for flocks under 30 birds. Small or large flocked, it shouldn't matter as you need the NPIP to sell fertile hatching eggs and such. So smaller scaled breeders should be able to get the testing. Here is the contacts for the people in charge of the NPIP, you can ask them who you contact for testing. Also try emailing your county agricultural agent.
USDA APHIS-VS
1506 Klondike Road
Road, Suite 300
Conyers, GA 30094
Phone

770) 922-3496
Dr. C Stephen Roney, Director, National Poultry Improvement Plan
Dr. Denise L. Brinson, Veterinary Medical Officer
Penny Kesler, Veterinary Program Specialist
Harold Perks, Veterinary Program Assistant
For me, its about $5 a bird but different areas charge different rates I believe. The idea is for the program to run itself and protect the birds, not make a profit. I am getting my flock tested after I bring in some new stock. Expect some paperwork, but your agent should tell you where to get it or send it too you. Send it back and arrange a date for testing, its a bit of a hassle but worth it if you want to show and sell some fertile eggs. Some states require record keeping as well. My state is much more lax than others.
Best of luck!