I think show breeders are more likely to do health testing, and screening because they are actually developing a line, and trying to improve the breed. Word gets out rather quickly if one of their dogs has a genetic illness. I would interview several breeders who are members of the YTCA, and find one who I could support. For breeder referrals see:
Yorkshire Terrier Club of America About The Club. I agree with Lou, check out the breeder forum for recommended tests.