What Randall said.
Texas is a big state. It takes 12 hours to drive from Houston to El Paso, with the cruise set at 85mph. I can't imagine that trip in a TR3 without cruise control.
I got a "vintage" off brand (Equinovox?) off ebay for under $20 still in the original box. It took a little to dial it in, but it works well. A club member got new units for his TR3 and TR4 and had to add a vacuum reservoir and relay on his brake switch, but he loves it. His control panel is held to the dash by magnets so it can be relocated behind the dash during shows. After driving across the Arizona desert, I had to add a pipe inside my vacuum line to prevent the rubber from collapsing, but otherwise, it has been reliable and extremely useful.
Jer