There is an article here entitled Guide to Finding A Reputable Breeder
It is a good one.
What I highly encourage is that you get the actual pre-screening tests done. All of them actually can not be done by a general vet.
1. Pra test - you can do either the genetic test - which will confirm for you if your dog is a carrier. Or go to an opthamologist who will check the eyes, and test for cataracts as well as Pra Progressive Retinal Atrophy. Then you post the results on the CERF database
2. LP evaluation - forms for submission are found at
www.offa.org
3. BATS testing.
4. HD evaluation this is an Xray and it is submitted to
www.offa.org for grading/evaluation
5. Expanded Thyroid Panel Post results on
www.caninehealthinfo.org
6. Hyperuricosuria genetic test forms found at Univ of Davis.
Responsible breeders do the testing!. I do encourage joining your local YT club or if a Toybreed club is closer join that one.
Also I would encourage you to go to local shows and meet your local YT breeders.
Also there are some good on line seminars on breeding, when I get the summary back from this organization I took the web in ars from I will try to remember to post back to this thread.