I purchased play sand from Home Depot for $3/bag.
I originally purchased the Ironmind sandbag kit, but I don't like it. It's not user friendly in the least. You'd be better off taking contractor bags, filling them up, Duct taping them closed and putting them in Army/Navy duffel bags.
Or, do like I recently did and purchase the new sandbags from Josh Henkin. I bought the new "medium" bag that can hold about 100lbs. With all the extra straps and handles, you can do a variety of exercises. I also ordered the small bag for my girlfriend.
We spent a night putting together a bunch of sandbags of various weights this week (~25, ~45, ~65) to go with my current 100lb bag (Ironmind). We used contractor's bags (doubled them up and Duct taped them shut). We'll be using them within the training program as a substitute for some "regular" exercises (overhead presses, clean and press, various squats, carries, curls, etc.).