if you start doing this a lot check local metal suppliers, metal recyclers, and craft shops. decent hardware stores often have a small selection with a large markup. there are 4-5 places within a mile of me where I can get copper sheet. for copper and aluminum sheet there are speciality distributors for roofing that you can get copper and aluminum sheet and roll from.
So after determining that there was nothing readily at hand nearby that I could readily identify, I turned to the internet. Whatever you think of their prices, Online Metals was willing to sell me the quantity I wanted for a few bucks, which fit my very limited budget. I could not afford to order bulk sheet copper and there were other reasons why it would have been a bad idea for me to do so.
A metal recycler would not have been acceptable. I wanted fresh, clean sheet copper. This was essentially an experiment, me testing how useful the antimicrobial properties of copper were to me, someone with a serious and incurable medical condition. Dirty/used copper would not have been acceptable for my experiment, which was medical in nature.
If anyone knows of another online source where you can order small quantities of copper and wants to chat up that they have better prices or better selection or something than onlinemetals.com, I am happy to hear recommendations. My condition is incurable and it's entirely possible I will wish to order more copper at some point in the future.
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”.)
I've applied it to some frequently-wet surfaces to successfully prevent microbial growth that previously had to be sanitized on a regular basis.
The adhesive keeps keeps it watertight to the surface, but the stuff I got could also be peeled off without residue. Because it's copper, it does develop a patina and leach green copper oxide onto towels when wiped. Brass tape may be a less-corrosive alternative.
People also use it for EM shielding, eg around electric guitar pickups.