They should be mounted about 0.5m apart, joists and ceiling strength permitting, and ideally the same distance from you as your side surrounds. Failing that I think around 1m behind you is the min if poss.
If you can box in the speakers with plasterboard or similar. Give them each approx 1 cubic foot of air, this will improve their bass response and help keep down sound leaking to the floor above. If you can get some Dacron filling (the stuff used in speaker as damping) put some in as well as this will also help with the bass.
You chosen well with the CCM80's, I've used many on installs mainly in kitchens.
I've only even used Dacron because we had an in-car dept at work, and they used it quite a lot for in-car subs etc. It's not really essential, boxing them in will be a huge improvement over not though.
For a supplier you could try a local in-car dealer, probably your best bet.