You could make (not a rangefinder but altitude telemetry system) one quite easily using a microcontroller, Xbee wireless modules, barometric pressure sensor and a serial LCD screen. Chuck a gps like the EM406A on while you are at it so you can get a second altitude measurement and ground speed.
Its sounds complicated but its actually pretty easy to do. Only thing is the bits would cost a few hundred dollars.

If you haven't used microcontrollers then the arduino with the xbee backpack would be a great place to start. Someone with experience could build the device and program it in less than 6 hours. For a beginner who knows but it is good fun in any case. Once you know how to program the microcontroller you can make heaps of things pretty easily. LIghting controllers, servo drivers whatever.

Paul.