Drills & multiplication tables

Time to start a new phase of tactics training ... multiplication table style drills.

We learn how to multiply with multiplication tables, but is there an equivalent in tactics? What is the 1 times table, the 2 times? etc... With multiplication we at least know the names of the tables, but how many of us can name all of the tactical themes we should be studying?!

So my plan is to first make a list of tactical themes, and then to learn each one MDLM style. How many themes are there? I have a copy of the Polgar book "Chess Middlegames" which has 77 different tactical themes, and it is probably not the entire list...

But, like multiplication tables, I will start with the easiest and work upwards. Lets say for the sake of argument that back rank mate is our 1 times table. So I went on to google to find examples of back rank mates, and found about 80 .. which is not really enough for any sort of MDLM training!

Does any one know of any good resources that give tactics by theme? Free sources would be a bonus of course! :)