I got one at Harbor Freight Tools a few years ago. I kept my '71 Javelin in it while I was planning and building my new garage. Works good for keeping the rain and sun off, but it won't keep out the moisture in the air. I had a fair bit of mold and mildew form on the interior while it was stored in there. It will be a lot wetter in there than if it was in a proper garage, so I don't think you should consider that a long term solution. I also had a mouse infestation and had to put rat/mouse poison in the car to keep them from moving in and destroying stuff.
Jeff
EDIT: I should have mentioned, my auto shelter survived hurricane Rita and Ike moving through Houston with no damage. After about 5 years the sun caused the nylon cover to get brittle and start to crack, but it worked pretty well up until then. Think I paid $150 at Harbor Freight, and I still have the metal frame pieces in my attic in case I want to use it again, I'll just throw a big tarp over the frame.