McMaster.com sells it, but of course the shipping is a pain because it is so heavy.

Lead shot in a canister of some sort works very well as you can add or subtract as needed w/ different drivers/wheels/fuel etc. A pc of pipe or square tubing w/ the ends capped and a threaded fitting on top will contain it nicely.

Melting lead is a bit of a pain and lead poisoning makes you stupid (literally).