I do this all the time. I usually use pork belly because I get a meat + lard. I cook the pork belly + seasonings in the crock pot on low overnight. In the morning you should have delicious belly in a pool of fat. The fat can be further purified on the stove.
I was just reading this site, http://www.cheeseslave.com/2009/07/09/how-to-render-lard-tallow/, which gives instructions for rendering fats in either the Oven, the Stove, or Crockpot. Haven't tried it out just yet...trying to source the fat first!
Yes. I prefer this method to using a stove because I get render the fat at low temperatures. Use the lowest setting possible. I could use an oven, I find that a little messy. (btw, I use Sanyo ECJ-HC100S model as my slow cooker. I love it. All the other slow cookers have warning stickers for lead in their ceramics.)
I have never seen pork belly in any market. Where do you get it?