Don't bother with a cheapie, get a decent Wacom that is the same aspect ratio as your computer screen.
http://www.ebay.co.uk/itm/WACOM-Intu...item35c49dab88
i.e. get a 16:9 or a 4:3 tablet depending on your monitor type.
Once you use a Wacom you will never want to use a mouse again, they really make a huge difference.