I have used several over the years. My favourite is Root Tabs by Aqua Pharmeceuticals but they are not available anymore over here. I have also used Dinosaur Poo which are ok, Aqua Master fertilizer tablets are ok and Dupla plant tablets which are good but expensive.
I am about to get some Seachem Flourish tablets which are highly rated but are again not cheap.
There are more available, have a look at some online aquaria shops or go to your LFS and give some a try.