where can i purchase the magnetic ring?

my tread mill would "bog down" as I was walking on it. Then it would go normal speed and "bog down" again about throwing me off the treadmill. My husband took the base apart and there is a black plastic ring that is on the right side(looking down to the right while standing on the treadmill) that goes around the metal cylinder that runs the belt. He says there is a magnet that is on this piece. Where can I purchase this piece?
I have a Sportcraft tx 4.9 treadmill modes 04041. The manual says to order parts at www.sportcraft.com, but we cannot find this specific model listed.