No, you can't bring it into a head shop in most cases. Phx uses standard parts so it does not need to be a PHX diffuser. Roors on the other hand have varying sized parts, so matching a roor part to any piece is a bit of a gamble. If you can measure the opening on the the broken part that might help, stems normally come in 12mm and 15mm, and rarely a few larger sizes.

You ought to go to the head shop or Internet site you got it from, if they don't have it they will know where to get it.