To me, the trouble with the cover that was linked by LeRoy, and with all the others I could find, is that they substantially alter the profile of the T3 seat. (This profile was shared with the T's, but is much sleeker than the G5's, etc.) I really like that profile.
I finally decided to take the cover and seat to a local upholsterer and have him use the old cover as a pattern for a new one. He cut up the old cover along its seams and then presumably traced patterns onto the new material. (I kept the cut up pieces in case I ever want to have another cover produced.) The result preserved the original profile, this time in leather instead of vinyl. The upholsterer was not able to reproduce the embossed pattern in the original, but that was less important than the shape, to me. The cost was a little over $100, ten years ago, FWIW. I've been quite happy with the seat since.
Moto