Someone was asking about pen grips earlier in the week. Attached is a photo of the one I use and swear by.
It's a rinG-Pen ultra which I got online but I can't remember where (my receipt has gone so cant give the name of the seller - they were UK based) so I'm sorry but you'll have to research it to find out more. It slides onto the pen/pencil and fits a standard biro. They come in different sizes. Mine is a medium and on the back it has 'rings tools.com patented' written on it.