Yes there are several, you have to be careful how much stuff you put back there depending on the height of your seat post.
They tend to be pricey. some of the bags mount directly to the cage mounts.
Go low with the water bottles and mount them to the seat post. And then.
http://www.all3sports.com/product_in...sign-aqua-rack
and is whats on my bike
Or go way out back and mount the seat bag underneath
http://www.all3sports.com/product_in...gn-rm-system-1
You can also talk to some of thee guys over in the tri forums and what they are using