Just threw out several sets of used one's. Check with Bill at ATECO. He does alot of 540 upgrades. Eddie Young too. Between them you could probably out together a whole set.
Also PM Tim Neckel (CFD5) He needed one or two last year- I gave him a whole box of them.
Those stock 502 pistons are mega tough. I had one that was rusted into a block. I beat it out with an 8 lb sledge and barely dented the underside. They're made by JE.