I work for a PCB manufacturer. All scrap has to be accounted for, and is
then sent for recycling (copper is costly). I've managed, over the last
couple of years, to get hold of some pieces of scrap, but these are few
and far between.

For cleaning boards, we use a sulfuric acid and pumice scrub. That last
part should be doable at home (I certainly won't be messing with reagent
grade sulfuric at home!). Maybe using previously mentioned lemon juice?

"Only two things are infinite, the Universe and human stupidity, and I'm
not sure about the former."
Albert Einstein