1. Try and find a set of 14" or 15" rims that fit your car. Yeah I realize that's probably impossible given the 4 on 3-3/4 bolt pattern. But you can buy them new since wheels and tires are not in the $500. And its racing you need the best tires you can get for that, um, fast, um, Spitfire.
4 on 3-3/4
Austin Marina
Triumph Spitfire, TR7
Lotus
2. Go find some guy with an old Lotus Elite/Elcat/Esprit and borrow his magnesium or aluminum wheels. Plenty of those sitting around in garages, not running.
3. Find a set of 14" universal fit slotted mag wheels, They are like the ones in #4, but have slotted holes and steel pieces that go in the holes
4. Get 4-8 of these wheels and have the holes re-drilled for your Triumph. There are usually some of these for sale in most cities in the USA. This would probably be the cheapest and the easiest. I think you can find these in 15" also, just look around for Datsun 240/260 wheels.
http://sfbay.craigslist.org/eby/pts/2028751976.html
Dudes Ex Machina:
https://www.facebook.com/dudesexmachina?Everyone who has ever built anywhere a 'new heaven' first found the power thereto in his own hell- Frederick Nietzsche