Accordian menu for side bar - recommendations plz.

Hi all, my side menu is huge due to the amount of categories I have. I need a menu that will tidy up the list and that I can have all my categories that begin with A, listed together and the rest the same in alphabetical order. So all the customer has to do is click the letter and it brings up all the choices beginning with that letter.

I hope this makes sense. Does anyone have any recommendations?

Thanks