You might try McMaster-Carr, which will (surprisingly to many) have a better selection of sizes and sell down to single, small cut sheets. Their prices are likely to be slightly better on a per-kg basis, which when coupled with less kg waste, could be even more savings.

I’ve also bought copper sheet from Aliexpress. What I got was copper sheet of apparent good quality. (I am using it for RFI/EMI, not medical, application so my tolerance may be wider than yours, in which case, McM-C is a more trusted supplier than “a rando on AliExpress”.)