Mike,
Search for "Space-Saver Spare Tire" posted by TR6Bill. That thread included comments and information on wheels.
As Alan said the bolt pattern is very common. It is 4 bolts on a 4.5 inch pattern. Datsun, Honda, Toyota, Ford, Saab and probably others used this pattern. The problem is the offset. Most of the recent wheels using this pattern are for front wheel drive applications. These generally have a positive offset, which puts the mounting surface out toward the face of the wheel. A negative offset (traditionally rear wheel drive) moves the mounting face further in toward the back or brake side of the wheel. I believe the TR4 used a 0 offset wheel. In order to use to use a wheel with a positive offset you will need an adapter. I've seen adapters for sale on EBay the price is something around $250. If you are looking for a newer style of wheel most likely you will need adapters to make them fit. The thickness of the adapters will depend on the amount of offset you need to compensate.
The additional cost for adapters combined with the cost of a set of used wheels from another car puts the price of this option in the range of a new set of Superlites.
Another consideration, if you are considering the purchase of used wheels, is the chance that the wheels are damaged or require refinishing.