Welcome! BMW doesn't have those turn signal lenses anymore. You can search on eBay for some, but you'll probably have to buy the whole mirror just to get the lens. Figure $80-90 and up, and good luck finding a right side mirror. They seem to suffer the most from falls as K-bikes seem to like falling on their right side.
My suggestion would be to just get rid of the mirrors. They fall off if you even look at them, anyway. Fill in the holes, and install some aftermarket turn signals and handlebar mounted mirrors. By selling the old mirrors you can probably get enough to pay for the new mirrors and turn signals.
A while back I saw a photo of an RS that this was done to and it really looked pretty good. Removing the big bulky mirrors and replacing them with smaller turn signals makes the fairing look a lot more sleek.
