Good question and an important one. One of my vets feels that many, if not most, cases of liver disease are caused by contaminated seed, seed that has not been harvested, dried, and stored properly and develops aflatoxins as a result.
A few years ago, I asked breeders I trusted what seed mixes they trust and most recommended Volkman's and Topper. Since breeders have their livelihood invested in their birds' good health, I feel piggybacking on their research and experience is a good idea.
After comparing price and availability, I selected Topper for my tiels. I buy it in 5lb bags for $11 at my nearby SuperPets, so there are no additional hefty shipping costs. Here's their homepage, but the descriptions of what's in each type mix aren't working for me this morning. Maybe soon. It's the Total Diet I use. The blend is safflower seed based, with no sunflower seed.
http://www.topperbirdranch.com/
Volkman's is probably the better quality seed, but only available on-line. Its cost is much higher since shipping is so high now. If I were feeding just one bird, I might give it more consideration.
http://www.finefeatheredfriends.net/volk.htm
In the past, I've been happy with McBride's seed mix w/o sunflower seed. I used it happily for years and switched only after having trouble getting the large bags I needed. It's available in 2-4 bags in good bird stores, so you might call to see who carries it near you.
http://www.naturechest.com/mcbridcoc.html
Paca