I never have buy special food for my cories, I just make sure that I give out enough food so that some sinks to the bottom. Of course you have to be careful with this kind of feeding, it is easy to overfeed. I have even seen cories swim up into the middle of the tank to catch some food floating down.
If you are worried about not feeding enough or over feeding, it won't hurt to have sinking pellets, your other fish will probably like to pick on them sometimes for fun.