Try one of the bag shops in your local shopping mall, I found my last two flight bags there, the first was a 25 buck bargain which got too small when I started IFR training, and the second one I found at a similar shop, it fits my needs perfectly and was half the price of similar aviation brand bags.
Just take all the stuff you want to carry and try it out in the shop, you'll know in a minute if you've got the right one.