Do you have a sewing machine, or know anybody that has one (and knows how to use it)? It's relatively simple to sew covers, I've done several using vinyl-covered upholstery fabric from the fabric stores. You must have a pretty stout sewing machine--like a 60 year old Singer-- for the heavier vinyl though, the newer machines are lightweight plastic and can't sew much heavy stuff.
Non-vinyl-covered upholstery cloth is easier than the vinyl stuff, though it's not rain resistant like the vinyl; blue denim material is even easier to sew but offers virtually no water repellency. Anything is better than nothing, though, and you can get a little water repellency by spraying the cloth with Scotchguard or something similar maybe.