I've found that there are a lot more icons in the original iBOX set(s) (there are 4 iBOX themes in that list that I've found) than the 3 packs on the market.
EDIT: I should be able to use Astro and move my market downloaded icons to my SdCard no? im going to try it.
Yes, though, you can get icons from pretty much anywhere and use them with BetterCut. One downfall I have found is that icons on your sd card also show up in your gallery, and if you have a .nomedia file in the icon folder, BetterCut won't find them.