Osmocote Plus is the one you want. Walmart sells it.
Having it in gelcaps is neater than ice cubes, because it takes the cap a couple hours to start dissolving (just a guess, dont hold me too that 2 hours but you get the point). With ice cubes, if you're not rather quick putting it where you want it, some stray balls will start dropping out due to the ice melting. The balls sink, so they will be frozen on the very bottom side of the ice cube, not in the middle. It's not really a big deal, just be prepared to have to push a few stray balls down as well. Also, the fact they sink means they'll be lying on the bottom, not floating around or anything, so it's not difficult at all to bury a few strays.
Lots of people sell ready made gel caps on *bay, I believe there may be a user here that sells them as well. Could try using the search function on the sale/trade forum. Ive always just made them myself or used the ice cubes