With the guidance of someone on another forum, I've finally sourced the black and white, and carbon fibre effect roundels for the E36 and E46 cabriolet (the boot badge is a different size and noone seems to sell it!).
These are from the USA, but the guy appears to have every combination possible, including wheel centre caps.
I've sent him a message asking if he would do a discount for a bulk buy. If so, I can get free postage here at work so would be happy to buy a few sets on behalf of others here in the UK, have them sent here and then send them on.
I'll let you know if he comes back with a positive response to the group buy.