I dunno for online shops, but you can always try to go to the local machine shop and ask them for some copper. They are not very fond to sell their stuff but when you tell them what a weirdo application you're trying to make... Also, they usually have leftovers you can usually use.
There also are places (scrapyards) collecting the metals for recycling. I bought some copper in such a place. No, it's not like digging in the dirt
, they usually have the metals collected and sorted in, you just pick your pieces, they put in onto the scale, pay per kilo, off you go...
Dunno if that is doable in your place, but you can always try...
Good luck!