The Rebels sure do have a lot of fancy gear. Even rebels need to be paid so where is all this money coming from. They set up shop on Hoth with a big fancy shield generator strong enough to protect them from Star Destroyer bombardment as well as an Ion Cannon big enough to bring down a SD. Sure they haves some old Y-Wings but those X-Wings look pretty state of the art. Someone has to be paying for the R&D as well as manufacturing of these things. Some of the stuff could be a part of a civilizations standard space fleet but the logistics of running this type of war would be crazy and very, very expensive.
The Empire probably is just pillaging each planet it comes across for materials and $$ just like the Nazi's did as they rolled across Europe. Do we really ever know who the big players are backing the Rebellion? What would they want as collaterial for there financial support? The US picked up a few strategic islands in the Pacific for example.