Basics

CuSO4
159.602 g/mol
Is somewhat safe, but it can be used as a algacide (for better or for worse) and exposure (most often by accidental injestion (ie keep it away from the kids) ) can cause copper poisoning
Is a medium "lightness" blue crystal (although some cheap sources may have darker blue and black impurities (these often show up in dissolution). This isn't of too much concern for our work though
Useful as it can be easily dissolved in water and electroplated readily to make copper-coated surfaces (even at low concentrations and very weak power sources) such as circuit boards on 3D printed objects.
Is hydroscopic, so store carefully
If pouring in bulk wear resperator and do so in a well ventilated area

Sourcing

Can be easily bought as either some sort of -icide (ie algacide, fungicide), or as a stump remover and/or root killer (Although these sources are a bit less pure than a chemical supplier
$3/lb - [1]

For copper plating of plastic, need to coat the plastic first with a conductor - (we could use a conductive filament as another option (see here Composite FDM Filaments )
[2]
Forum thread, useful suppliers - [3]
Electroless by a vendor - [4]