I echo the places already mentioned, especially Open Source Seed Initiative.
Pinetree Garden Seeds sells packets of mixed seeds, for example a packet might contain 6 varieties of radish seed. I'm sure other seed companies have similar offerings.
I participate in a lot of tit-to-tat seed swaps: Send in some seeds, get some seeds back. I end up paying postage.
Peace Seeds, and Peace Seedlings are great sources of genetically diverse seeds.
Carol Deppe's Fertile Valley Seeds has some offerings with a lot of diversity. For example, she sells my muskmelons and moschata squash. (I have test grown my varieties that she is selling, and they passed my QC checks with high scores.)
Some of my
local farm stands
sell genetically diverse melons, and squash. I go through the bins and select the "off-types" and add them to my landrace breeding projects. I add seeds to my landraces that I collect at the grocery store: beans, melons, and squash are great. A package of 15 beans soup is a great way to trial a lot of varieties of beans at a low cost.
If I plant a couple varieties this year, and a couple more next year, and a couple more the year after, and save/replant seeds each year, and allow them to promiscuously pollinate as much as possible, it doesn't take long to generate a lot of diversity.
The thing about growing landrace varieties, is that I don't care about purity. Other than in the broadest sense, for example, not adding hot-genes to my sweet peppers. So pretty much any seed I receive from any source can be planted in a semi-isolated location, and if it grows OK, it gets added to the landrace.