Welcome Aboard !

• Can you please tell us where the cable broke ?

• Handlebar company Flanders was at one time making aftermarket cables for BMWs.

Does anyone make an after market clutch cable for 1991 R100GS? The new ones BMW sells are crap. I had two break in under 10,000 miles.

I don't have a GS, but I've had excellent service from stock BMW cables. At roughly $20 a pop, they represent a very good value. In my expeiernce control cables, especially clutch cables due to their higher tension, need special care during installation, including....

Lubrication
• The primary point of breakage is just where the cable enters the clutch lever. If the barrel can't pivot in the handle, then the cable goes into bending, which it was never designed to handle. Cables are only designed for pure tensile loads. Therefore, filling the barrel void on the undersided of the lever with grease BEFORE installing the new cable is mandatory. If the clutch cable broke at the clutch lever, then that is 100% installation error.
• Lubricating the cable itself. A little ATF never hurts.
• Lubricating the lever pivot point. I like to use motorcycle chain lube on the pivot becasue it soaks in, and then sets up as water-proof grease.

Routing
• Correct routing with large radius bends can greatly reduce the force required at the lever.
• It's not enough to follow the old cable path. If the new cable is 1/2" longer or shorter, then the cable might need a different routing.
• Cable routing should never ever include hard strapping with cable ties. If you want to use cable ties to guide the cable fine, but you should be able to fit a finger inside the cable tie after installation is complete.

All the best.