I use one from Argos that cost about £40 (made by Trust), but have heard the Wacom ones are even better (and can be picked up from ebay as can be more expensive to buy).
I also find the mouse a bit more difficult to use on the one I have (mine came with mouse and pen) so tend use the pen all the time instead of the mouse now for most things (whether its drawing related or not) I do on the PC as its more comfortable.