Fedco and other nurseries are reselling what they buy from the wholesale growers/distributors. So what you buy from Cummins likely came from Willamette or Treco. Most all carry B118 and M111 but if you want something else there might only be one or 2 places to get it. If you want Polish 18, you'll only find it at Cummins or Treco. Cummins and Willamette sell the most Genevas.
If you want 50 of something, look at Willamette first to see if they have the variety and caliper (diameter) you want in a bundle of 50. If bundles of 100 or you want 50 each of 2 things, look at Willamette, Treco, and Copenhaven. If you want seedling rootstock, check out Lawyers too.
If you want 25 to 49, Raintree has a nice price break at 25.
For smaller numbers, whichever retail nursery has it and is most convenient.