Attila I got 2 black rexes from rescue earlier this year - I gave them £80 although they only asked for £70 - both neutered and myxi vacc'd.
They are probably not quite show standard - a bit of brown in their coats but that might be because they spend alot of time outside and it might just be the sunshine.
I'd agree to ring a few rescues to see if they have any but there is a british rex rabbit society and some specific colour rex societies aswell if you want more info on the breed standard. I bet there are quite alot that end up in rescue because they don't meet the breed standard, as its very specific.
I would think that you'd be able to tell from their coats if the ones in the pet shop are crosses or not - theres nothing that feels quite like a rexy coat