I've used a couple of the cheap neoprene ones from the fitness section at Walmart. The really cheap, blue one (I think it was about $5 to $7) was too flimsy and tore the first time I used it. They also had a red one for about $10 that worked pretty well. I warn you, however, that they are hot and bulky. If there is any way you can swing it I would highly recommend saving up the $30 (plus shipping) for an Underworks binder. They are so much more comfortable and less obtrusive. Worth every penny, IMO.
P.S. If you Google "Underworks Coupon" you can usually find coupon codes that will save you 10% off your order.