We put out a call for empty tidy cat jugs, and we fill them with a grain scoop and funnel to give to our fosters.  When fosters need a refill, they only get a new jug of litter when they bring back the empty one.  

Suggestion number 2: Pine Pellets.  Find a horse feed/tack store and see if they sell pine pellet bedding.  Same thing as the pine pellets marketed for cats, but a fraction of the price.  I think we were paying $7 for a 40# bag, but that was a few years ago.  Regardless, it was a fraction the cost of Feline Pine or an equivalent.  And like the super sack, if you buy a pallet at a time, they might be willing to deliver.

Suggestion number 3: If there's a big box store in your area, like Target or Walmart, talk to them and ask what they do with their breakage and damaged pet products like litter and cat food.  An open or damaged box/bag of litter can't be sold, but it can be donated to a rescue/shelter instead of thrown out.  Same for opened/damaged boxes of food.   Again, there will be some work (you'll have to send someone on a very regular basis to pick this stuff up) but it can turn into a *lot* of free food and litter.

Oh, and if you do wind up buying multiple (normal) bags of litter or pellets in large quantities, ask if they can give a discount, especially if you're planning to continue purchases going forward.  When you buy stuff a pallet at a time, they're usually willing to give you a break.  (The worst they can say is no, so ask!)

Hope that helps.  Please let me know if you have questions.

-Chris