ebay is where you will likely find the best selection. I am not sure this is specifically for your model, but the E60 is very close to the E70 with iDrive and so forth.
New Optical Laser Lens Pickup for BMW E60 E61 M Ask 2 Navigation Unit | eBay
Another example, again you will need to verify fitment -
New Laser Optical Lens Pick Up for BMW MK4 Navigation DVD M3 5 | eBay
I have had to replace laser in BMW rear seat DVD entertainment systems and found that there were no good listings on ebay, but found numbers and did research and found it was pretty much the same laser that is in many of the DVD Nav drives.
Often you can find some reference numbers on the current laser, find DIY's with info and do your own comparison using
BMW Parts Catalog for the main module part numbers.
Often the ebay descriptions are more generalized because there are SO many different BMW models these days. Also keep in mind there have been a number of CCC iDrive changes, but many may not have impacted the Nav laser.