One of the biggest mistakes i've made as an artist was to do a series of paintings on huge masonite... and then try to find a way to hang them. Unless you have a woodshop and some building skills, they can be very tricky to move, build out, keep from warping and hang. After all the trouble, I just wished i'd saved up some Michaels and Blick coupons and worked on canvas!
I buy most of my canvas at Dick Blick Art Supply, and i've developed a great relationship with the owner and workers of our local shop. They send us 20 - 60% off coupons and have at least 4 canvas sales a year. You can also get a 40% off one item coupon for Michaels from the Sunday paper and just buy one canvas a week. My Michaels carries them up to about 3x5.
If you don't have that sort of store near you and you are really jonesing to paint big, you can always paint the walls or paint huge raw canvas. If you paint on raw/gessoed but unstretched canvas, you can always staple it to the wall, have Home Depot or Lowe's cut baseboard to size and just nail your own frame directly to the wall around the canvas. I'm a muralist, so I just don't think there's anything as lovely as a wall to paint on... but you're pretty much stuck with what you've got for a while. Or you're stuck sanding, priming and repainting.
Lastly, try going to Goodwill or a thrift shop and buying old paintings or artwork that is on sale there. You can prime over almost anything, or you can find something with a nice frame and glass and put works on paper in there. I got a hideous painting of a ceramic jug for $4-- and it was a 2.5x4 canvas once it was gessoed.
Good luck!