For a start try the sites that show up here
Also check out Bluebee's best links thread on page 1 of this E39 forum. I'm sure there is a section on recommended vendors.
Also search the best links thread for transmission for help with diagnosis. You don't sound certain of the sensor being at fault. BTW, limp mode will feel like a lot of slip. Slip mode defaults to 4th gear so the torque converter does need a lot of slip while pulling away from a stop.