You can get hte numbers from most motor sport suppliers such as Demon Tweeks. You just stick them onto a white square of plastic. But driving a car on the public highway with numbers on the side when not competing in an event is illegal. It will at the least give the police reason to stop you just for a chat.
The White square to put your numers on you can make from a piece of sticky backed plastic.
As for rally plates they are only available from rally organisers as far as I know. And then only if you are part of the organisation or competing, although occaisionaly marshals can get hold of them. Apart from major events these do not have the big white sqatre as part of the plate. Thea sre just the bit over the top.