Usually, places like Cabela's and Bass Pro Shops are good for that sort of thing online. Surprisingly, a lot of local fishing shops will carry a limited supply as well. I've seen some sold on the Classified page here every now and then.
I believe that "pouring your own" might have been more popular in the past because jigs needed for steelhead fishing used to suck many phalli at once. The hooks were virtually worthless and, with the lighter ones, the hooks were way too small. With the popularity of jigs in the recent past, there are now good jigs and jig heads available. Like everyone has said, Rvrfshr products kick ass, are very well priced, and are created/sold by PP board members that have contributed a lot (too much?)to the board over the years. Besides that, if they give you something that sucks, you can give them hell right here.
In the end, you'll probably buy a bunch of jig heads AND get a mold. You got bit by the bug hard.
If working with lead, do it OUTSIDE. Also, others may disagree with me but 1/4 oz is getting on the heavy end for steelie jigs. It seems like a lot of guys are fishing very light jig heads. I'm not really sure what is "normal" though?
-Kaiser D.