Its easy to find out if BMW sells a replacement piece...it would be in the part database diagram for the remote. If its not there then the only thing left is the aftermarket purveyors that sell aftermarket remotes like you've seen on ebay.
Go to one of the online part database sites like
www.realoem.com .
www.BMWfans.info. or
https://bimmercat.com/ ...enter the last 7 digits of your VIN then work your way to. the remote key diagram.
EDIT: Curiosity made me look it up...the insert is NOT sold separately by BMW. Looks like you have to purchase #2 in the diagram below..(which is cheaper than buying #6 which includes the blade insert key & the module.