^
Just go to any store (Walmart etc.) and shop for parts that are close to what you want then modify them to fit your needs. If you buy "motorcycle" bags out of a catalog then you'll pay 2x or 3x as much as the same type bag (say a nylon travel bag or camera bag) just because it is a "motorcycle" part. Same with racks, I have modified other things to fit, things I have found dirt cheap! Now if you want fancy leather saddlebags etc., well that's going to cost you!
I bought this bag at a garage sale. I think it is a camera bag of sorts. I modified the strap and repositioned it, then made custom mounting straps (w/ buckles) for the back seat. So now the whole thing can easily unbuckle on and off the bike. With compartments and enough rooms for my tool kit and bungee cords, and perhaps something the size of a textbook.
I have modified nylon lunch bags, nylon travel packs and many other things to work in the past. Many better (especially in exact size, fit, and cost) than store bought. Total cost, 50 cents, total time, an hour. Just use your imagination!
http://img.photobucket.com/albums/v287/WingZero721/02.jpg