i dont think it's comprehensive myself, but finding an unused id was pretty easy. basically, in col's rfa explorer, i open up GAME.rfai chose "Search RFA", then typed in a number (like 853 into the second field), and see if anything is found. if somthing comes back, i go check em and see if that number is a material id.
looks like 853 is cluster bomb.
another way would be to copy and paste everything materialManagerdefine.con into notepad, and just search that text file for number.
look like 854 is free