I found a velcro-on one made by TwoFish.

http://www.boomerdirect.shoppingcart...53/1226148.htm

^^There it is, I have no idea why it's $20, my LBS sold it to me with a bottle for under $5.

Or you could try a tri-style one that mounts on the seatpost (such as Profile Aqua Rack) and still get a saddle bag above it.

http://www.rei.com/OM/style/721789?c...:referralID=NA

I use both depending on the conditions. The Aqua Rack looks dorky, so I only bust it out when I need 2 bottles. Otherwise it's the velcro on, or a small bottle in the jersey pocket.