You actually get a 3-byte prefix from the IEEE (and it'll cost you $1200 - that's $50 per bit!). They won't give you another prefix until you've used at least 90% of the one you currently have.

Making up MAC addresses is generally fine, as they stay on the local network and there are 2^48 of them, which is quite a lot... you have to be careful though, as some of the early bytes (2nd byte, bit 0?) signifies things like multicast...

Hugo