why do you need/want extra dice?
Who would't? It is dice game and if you're reading this forum you'll probably like it. If you like dice game, then you'll probably like dice too. And there can't be too much dice. Amateurs say too many, professionals too much.
Elder Sign can be played with seven friends (cultists/victims etc.) and at least you need two sets, one for you and one for them. Of course true fan has eight complete sets of Elder Sign dices for that magic evening when he isn't playing solo, when the stars are right. At least he is a little bit crunchier feast for Cthulhu with all those dice than his poor neighbor.