You could just buy them, but that gets to be expensive. Instead I would buy gelatin capsules in size 0, and fill them with Dynamite Plant Food (the kind in a red bottle). They can be purchased on Amazon for under 15 dollars, and that gets you 300 tabs (the capsules) and i think 2lbs of the fert, which is plenty for a while. The one problem with doing this is it risks ammonia problems, since the ferts are meant for use on plants in pots.
For more info on these tabs, just google "DIY root tabs" and there should be a thread on this forum (where I got this recipe) and for The Planted Tank (I haven't looked at this one yet, but I'm sure it's good.