You have a few options;
You could go for a OEM bumper, either new from BMW or used from e-bay. On ebay, just do a search for BMW F10 M sport rear bumper and see if there are any for sale. You can also find diffusers there. If you're lucky you'll find a bumper in the same colour as your car.
New bumpers can also be sourced via aftermarket vendors, but fit and finish might not be 100% as good as stock.
Here's a few links for you:
M5 Bumpers:
http://f10.5post.com/forums/showthread.php?t=726049
Here's one for sale on this forum:
http://f10.5post.com/forums/showthread.php?t=903373
Complete parts list for OEM bumper conversion:
http://f10.5post.com/forums/showthread.php?t=848376
All of the parts in the last link can be bought via your local BMW dealer. You can also go on
www.realoem.com and find the part numbers you need if you want a M5 diffuser etc.
I have a M5 diffuser and M-sport rear bumper, you need the parts listed and need to remove a plastic cover behind the bumper. But a really easy DIY.