Beth:
I don't have a supplier to direct you to, but I have a couple of suggestions that may help.
Try contacting Trekell brush company and see if they could build you the type of brush with a thicker handle. They are great folks and have made special brushes for people before - and their prices are very reasonable.
www.trekell.com
Second, have you tried things to thicken the handle yourself? There are rubber grips you can buy at office supply stores that slip over things like pencils and brushes to make them easier to hold. You could do the same with clay or a kneaded eraser molded around the handle.
I am sure others will have suggestions too, but give those a thought.
