Go to your nearest concrete plant and get a load of everyday sand that they use in making concrete. That's all you need. It won't be entirely uniform, but it will work perfectly. You need to figure out how deep you want it.
Assuming 6" deep your problem will look like this 6"x72"x144" = X Divide X by 46,656. ( cubic yard = 36x36x36) That will tell you how much you need. Increase by 25% so you have some extra.
Your final calculation would look like this. 6 x 72 x 144 = X. X/45,656 x 1.25 = what you need. Adjust for depth by changing the first number. Here's a hint. You'll need a trailer or pick-up truck that can hold about 3,000 lbs of weight, or you can make a few trips.