I have several 10 mm's including Glocks, its a great caliber
BUT if this person has slides, he should have had the frames too, never seen them sell just slides for 10 mm
and I was a Glock Gold stocking dealer for 10+ yrs, THAT or you could TRY and find a used Glock 10 mm and then have a spare slide and be ahead of the game
OR try again contacting Glock and ask then if they would sell you JUST a frame, as that is the registered part of it??

I blew up a Glock 17(first gen)once, and sent it back to Glock, and they gave me a NEW frame a 3 rd gen frame, and same old Serial number, to correct things for me.
they had awesome customer service when I dealt with them, but that was 15+ yrs ago, customer service has changed a lot in many company's since then, but I would TRY< never hurts to ask!
and might be surprised what they Might do for you, as never know, maybe there still a top shelf service company??