Cheapest bet is to keep watching eBay for parts that others bought but then gave up on. Also, if you can find a solid shell for a parts car you can harvest donor panels, although it can be a lot of extra work. Bolt-on pieces are easy.
Floors can often be patched with flat sheet steel from a hardware store if you are not a bug about authenticity.
The drivers side floor pan of the MGB I drove in college was the door skin of a Buck Electra that I harvested from the junk pile behind a body shop. It was crinkled but not torn, and we stomped it flat before cutting out what we needed. The passenger side floor pan was the cover from an old oil furnace I found at the junkyard.
It wasn't elegant but it kept it on the road!