They can also be made yourself with cheap parts. Some people use electic scooter motors and a speed controller. Put it in a box and put an arbor on the motor. And your good to go. Check on youtube or this forum.
I brought my diamond grinding disks on Ali express. They are good disks, and even if they wear out, it wont cost an arm and a leg to replace them. I have them in 240, 600, 1200 and 3000 grit.