If anybody's ever seen any of my posts before, they'll probably realise I'm a stingy bugger and will never shell out for decent instruments.
I needed something to gig with, so I bought a crappy Encore P-bass copy. for £30, it's actually not bad (I originally bought it just to salvage the neck but never got around to it... but that's a story for another day).

Anyway, I'm unsure as to what kind of pickguards I can get for these seeing as it's a copy, I'd assume they're not standardised or anything.
I'm looking for a black one, with 12 holes.

Will any old 12 hole pickguard do?
Your probably going to have to buy a Fender replacent pickguard and then drill new holes on the body where the screws are going to go.

The other option you might want to consider is painting the existing pickguard. Pop it off, gently sand the face of the pickguard and hit it with a coat of spray paint.
Your best bet would be getting a sheet of acrylic, tracing the shape of the original pickguard, cutting it out with a small hacksaw, drilling the holes and smoothing out the edge with a file.