You can research dog foods at
www.dogfoodanalysis.com
This gives a lot of information, but it's not THE only resource out there. I encourage you to do your own research before choosing a dog food, and then you may need to try a couple of brands before you find one that works best for you and for your dog.
When you do get onto that site, look for 4 and 5 star foods-the 6 star foods are too rich for yorkies, and are better suited for large working breed dogs.
I home prepare meals for my dogs-I feed raw meat, plus a grain and a vegetable, with supplements mixed in. I also give treats of yogurt, cottage cheese, cheese, and fruit. If you're looking for a place to start homecooking, check out "Dr. Pitcairn's Complete Guide to Natural Health for Dogs & Cats". I use this, and so do many other YT members. As always, please feel free to PM the homecookers-we're all happy to help! I started about 6 months ago, and I relyed heavily on my fellow homecooking YT members
Everyone was really helpful!