Can you always find squash blossoms? I don't see them.
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In regards to farmers' markets, you'll only find them when squash are in season, just before they've bloomed. That's a tricky answer, though, because you see butternut, and other hardy squash varieties way into the winter here in the northeast. That's because those types last months just sitting on your counter.
So, the real answer is you'll find them just until the last of the winter squash begins to bloom. As for supermarkets, I've seen them in high-end gourmet markets year-round, but they're being flown in from who knows where, and come with a very high price tag.
squash blossoms http://www.thecolorsofindiancooking.com/2010/07/squash-blossoms-edible-flower-and-how.html
They are extremely seasonal. And have a very, very tight window of availability. If you want them locally. Keep your eyes peeled and be prepared to buy them quickly.
If you come across them, make sure to inspect them carefully before purchasing, as they do not keep long once removed from the plant in my experience. They really are a "food of the moment."
Some produce suppliers will source them and fly them in, but you'll pay for the air freight, and probably have to buy a box of them...
They wilt quickly and are tough to find, so its best to try to grow your own. Easy to do in midwest from May-October. Use the Romanesca variety. they have the best flowers. Greg