Either thisDo you buy bulk carbon? It's $15 for 10oz here.
Bulk Aquarium Large Particle Carbon - Calgon (5 Gallon - 15 lbs)
Shop The Best Saltwater Aquarium Equipment & Supplies at Saltwater Aquarium. We Carry Everything you will Need for your Fish Tank.
www.saltwateraquarium.com
Or I stop by the local Rainsoft dealer and buy it a cubic foot at a time lol. About 100 bucks for 27 or so pounds. But they do not always have a bag in stock.
Edit: The bulk in a bag has a lot of dust to be washed off, as they trhow the bags around when loading/unloading.