I have these:
Home - Truck System Technologies, Inc
buying them direct is about the same price as from a dealer, so I'd buy direct. Also, I carry a spare sensor or two so if I'm on the road and one acts up I can change it out right away and keep rolling, and send the bad one back at my leisure. Generally pretty reliable, but I had some of the earlier versions that went bad (that they replaced under warranty, no questions asked), the new versions seem better. Overall happy with the brand.
They will tell you you need the signal booster for longer units, but I have not needed it. No problem picking up the sensors on a 40' trailer with just the unit hanging on the dash.