Are the P53 shirts rare and hard to come across? I've only been focused on P41s and P44s and I don't think I have ever seen any P53 fatigues before.
I have been combing ebay for several months and the P53 shirts aren't exactly common, but there was generally at least one or two for sale at any given time (you might have to jockey around with your search terms). However, I've only seen the trousers legitimately on sale once. There are a few listings from vintage clothing sale accounts that seem to deliberately misidentify these as WW2 era P41 trousers and overprice them accordingly. Same situation with the later cotton sateen P56 trousers, the shirts are available, but the trousers rarely seem to come up comparably.