I used a few different leather bags on mine, but I think my favorite so far were cordura, and cost me $25 from JCWhitney.
LinkThey were cheap, they carried a lot more stuff than I thought they would, and they handled the weather better than the leather ones ever did (although if you carry anything in them while it's raining, it WILL get wet if you don't bag it). I've seriously considered getting another set to replace the ones I sold to put on the leather ones.