Where can I find squash blossoms in Washington, DC?

Heinz' farm is Next StepProduce, which should be easier to find than trying to figure out who's the Swiss farmer!

At the Dupont Circle farmers markets on Sundays, you should find them. New Morning Farms usually has them, and I think Heinz, the Swiss farmer whose farm is in Southern Maryland, has them too. I think you are a bit early for squash blossoms, although the season has been so screwy this year, who knows.

I agree with ReneePussman, The Farmer's Market is the best place to find them as the blossoms age quickly in any sort of storage. It is best to use them the day you harvest them. Talk the farmers that bring squash to the market as they may not have brought blossoms but they may be able to make arrangement to get some to you. If you have any green space around your abode there are many bush varieties of zucchini and yellow squash that do not take up an excruciating amount of space and yield copious results.
good luck

Farmers markets and specialty stores are usually the best place to find item like that.