Does anybody know where i can rent those huge white backdrops they use in music videos where the backdrop blends into the ground? What is this called?

What size do you need?
Most aren't rentable. The narrower ones are long paper rolls that go from the wall to the floor seamlessly. Big, huge ones are either stationary or built for the job--often called cycloramas. They could be fabric or plywood on a frame of some kind.
You may be able to find backdrops for rent--but they'll be textured, not white. I'd start at a well equipped photography store/rental house in your area.

or u culd just paint a wall white.

This is going to be used for a music video, so it has to be fairly large to accomodate the band and that wide open look. (I'm going after the look when there is white evrywhere you look in the room). I will be using dollies and cranes so i need white to at least outline the band and the rest i will do digitally.

couldn't you just greenscreen it, then replace it with white? i'm not sure if it'd have quite the same effect though.

I could just green screen it, then change it to white... but then i would need a huge green screen then, wouldn't I? And if i'm gonna use a green screen to change it to white, why don't i just use a white screen in the first place?

Green screening wouldn't look nearly as good as a real backdrop. I wouldn't recommend that.

I found out the name of what I'm looking for, but I can't seem to find any big enough (at least 40 feet across). It's called an "infinity backdrop, or an infinity background".Any suggestions on how to make one or site url's where i can find them?

you might be better off renting a stage with a white cyc.

maybe talk to a theatre or lighting hire company, they usually carry white curtains, and they might be able to help.