Could you not ring the local tack shops and feed merchants in that area, through their local knowledge hopefully a long list of yards will come flooding through.
I have always found the farriers to be the ones in the know and you could always get hold of a farriers number and ask him to mark your card. They are at the yards on a daily basis and will be able to tell you the good from the bad. Personally that would be my first port of call.
You could try asking at local riding schools or do what a friend did she just drove around and asked at places that looked like a livery yard(you know lot of horses in field,signs jumps etc)Or you could see if local riding club knew of anything.
Ooh just thought why not ask your vet or farrier as they are bound to know some.