Are you referring to the carousel ring under the glass tray? Or, are you referring to the turntable coupler in the center of the oven where the tray rests? Both these parts can usually be purchased on line at any of the following websites:
All these sites are reputable and have great service. Prices vary between them, so shop and compare. The first three sites I have listed include helpful exploded view parts illustrations to assist you in locating and properly identifying any parts you may need.
You do not see the turntable shaft in the center of the oven cavity rotating, you may actually have a turntable motor that has failed. The motor is attached to the center plastic piece (called a turntable shaft or coupler) under the middle of the glass tray. The following link explains how to access and replace one:http://www.fixya.com/support/r3555716-replacing_a_turntable_motor_on_a_microwa
This is a very simple repair, that usually is not expensive. The price range will be determined by the model and manufacturer. This repair can also be performed on an over the range model microwave oven WITHOUT uninstalling it.
If you have any questions, or require additional assistance, please post back with your MODEL NUMBER. I hope you find this information helpful.