API used to sell them but I don't think they make them anymore. You might still find some though. Dennerle do Deponit Balls which are supposed to be good but expensive and not many shops stock Dennerle products.
You can use anything as long as it doesn't contain any herbicide or phosphates. In other words, the middle number must be zero. Ideally, you want KNO3 for root tabs. KNO3 fertilizer is 10-0-10.