I would not buy a bolt of broadcloth without first getting a swatch or small piece to check it out. There are some great broadcloths (Moda) and some I don't like (Kona is too heavy for me) and probably some that are really tacky.
If you are looking for muslin, I would wait for a big Hancocks of Paducah sale (unfortunately, only once or twice a year). Kaufman Kona muslin is lovely quality (snow is a nice cream color that is not too white and not too cream) and comes to maybe $2.50/yard during this sale for the 44". It also comes in wider widths if you prefer those for backing fabric, with similar savings.
Otherwise my usual go-to place to shop is
http://fabric.com . Have never purchased an entire bolt from them, though.
I think if you want 20 yards of anything, you need to make sure you really like the fabric first. There is tremendous variation in feel, thread count, handling qualities, etc.
Edit: For the JoAnn's fabric, if you have a store nearby you can go there and see if they have the fabric you are thinking of ordering. The stores often have the same fabrics, although not always at the same prices.