Old Man Mountain is about the only solution for you.

http://www.oldmanmountain.com/

I'm not sure what "waterproof sacks" means in terms of capacity, fit, attachment and most importantly, safety. Most decent racks are designed to support panniers, which entail a lot more necessary design elements than a sack. You can not easily fix a stuff sack to a rack securely except over the top, which limits the volume and mass you could carry in the sack. Anything you attach to a bike in the vicinity of rotating wheels needs to be done so in a manner that is really secure, if you value your teeth/collarbone/general looks. If you're going to invest in decent racks, it only makes sense economically and practically if you go ahead and add some panniers.

Otherwise, consider a trailer - Burly Nomad, Bob Yak, and several others to chose from. All of them cost about the same or less than good racks+panniers, and the more popular trailers are durable enough that they can be used and then resold on ebay for half their original cost, or more.