My opinion as a gardener, planted tank keeper, ignorant of paludaria:
You could check out gardening/building supply centers.
They generally have stone far cheaper than LFS.
But, as AT says, be sure you know what you are adding to your tank.
Realize that if you are mounding stone, only the outermost/topmost layer will be visible, and much of that will be obscured by plants. So the majority of your fill, and even a surprising part of the outer layer, can be comprised of relatively unattractive material. Even something like brick. Don't spend too much money on neat looking material that will be invisible in your completed environment.
You can even use inert inorganic material, plastics, pvc, or foams, to make up the "bones" of your topography.