I have seen (and bought) some Food&Wine Festival merch shortly after it ended. It will be leftover merch, and shirts/sweatshirts may be in odd sizes. Saw cups or glasses too.
I'm pretty sure we bought Illuminations Farewell plastic straw cups after it ended and Epcot Forever had already started. They were on sale, so that's why we bought them.

Any leftover merchandise that we have found has been in MouseGears, on a display near the cash registers/line near the event/passholder merchandise display racks.
Things like event pins or ornaments tend to sell out quickly, so I don't think you'd find anything like that leftover.