I have a 68 convertible (very similar) and I can tell you that anything having to do with the instrument cluster is going to be either 1) insanely expensive on eBay or 2) not reproduced. Check out Dakota Digital. All you would need is the bezel that goes around the instrument cluster and the ability to wire it up. They are a little pricey, but I know they make one for a 67. On my list of things to do this winter.
The rear seat spring assembly should be available. Try Year One. Would just have to have it upholstered. As for the other things you're looking for, check out Desert Valley scrap. They pick and ship stuff off of their huge inventory of rust-free Arizona cars. About your best bet. Welcome to the world of oddball year impalas. They can be frustrating, expensive cars to restore, but I wouldn't trade mine for anything else on the road.