They’re no longer available from BMW and were never part of the M kit.
They tend to sell for a lot of money on the rare occasion when they come up, but strangely cars sold with the full aero kit don’t seem to command a premium in line with the cost of parts.
The cheapest option may be to find someone selling a full car with the kit and swap them for yours, then sell the donor car on. Or strip the whole car as the bumpers get a reasonable price too.