Welcome to MAAST!
If you are looking to save money, try using clear acrylic rod and drilling holes in your existing rock to 'piece' it together like you want and keep it from shifting. You might also look into some dry rock (Reef Bones or the like) as it is less expensive than live rock.
I don't know any LFS that would simply trade their rock for yours pound for pound. What's in it for them?
Custom Acrylic 180g RR, acrylic sump, 45BR frag tank, 18g fuge, 250+/- lbs LR, Euro Reef CS8-2, 10x39W T5, softies, LPS, and a few SPS.