I've looked a bit for such a commercial timer, but they're indeed hard to find.
It is possible make one using less as 10-12 very cheap components, but problem (dunno your experience) may be designing it and building a safe piece of equipment, assuming it will work 120/240 volts AC.
Personally I've been playing with electronics and micro-controllers for a while and I guess loads of hobbyists with a bit of experience could make one.
Probably loads of other forums have a similar sub-forum, here's one on the arduino-forum, "gigs and collaboration" where you could search for a "freelancer".
http://arduino.cc/forum/index.php/board,26.0.html
http://forum.sparkfun.com/ and
http://forums.adafruit.com/ may also be good places to find a hobbyist who could do it for you.
Simplest might be a timer that's variable, meaning a 3 minute cycle and one knob used to set the time power is on during that time.
If you're looking for that (cheapest) tell a friend of yours told you something about a 555-timerchip and pulse width modulation.
If you would like something with an LCD-display and the possibility to edit all parameters a solution with micro-controller may be better, but extra parts and coding will probably cost more.
