Depends how wide it is - garage doors in steel are normally made to a specific size/s, so to buy one and then find it does not fit - would be a waste.
Steel doors can be bought off Ebay for very little - just make sure you get all the runners and cables.
Wooden doors - you could make them yourself - just make sure you have good solid cross bracing to add strength - you could probably make a set for around £60
