I bought some on eBay with success; yes eBay has its issues, but many sellers there are pretty good.
Note that there are (at least) 2 versions of the 12ga action bar, they differ in the way the operating lever fits in. There are old levers, then different ones, then the later ones which are universal. One type has a notch on the edge, one type has a dimple for the detent, and the universal ones have both. So the action bar you can buy depends on which lever you have. If you can find an action bar with lever included, that saves you having to deal with this issue. However it is usually best to buy a new action bar, the used ones start to fatigue with use obviously.