Well you buy it in a garden centre and there are several varieties, might be best if you explained to someone on the staff what you are needing for.

Personally, I'd use compost from a growmore bag. The big problem you'll have is keeping sufficient moisture in, so what I'd do is line the basket with plastic (I use old compost bags for my hanging baskets - just make sure you get the black side outside and the pictures inside) and just put a few holes for drainage around the sides (not in the bottom - thereby the water will pool a little in the bottom of the liner and provide a sort of reservoir). You could also use water retaining chrystals, but I've never used them personally. You'll need to provide your toms with all the nutrients they need (the nutrients already in compost dont last more than a few weeks whatever sort you use). So, you'll need to feed your toms either weekly or with some brands it's every time you water, which will probably be every day. Hope this helps.