I do have thin fingers, as did my mother -- we also have long fingernails.
I had a child's thimble and it fit but was on the 'short' side, still it worked for years. My mother had an old one as well. then I found antique thimbles. You can often find them at antique shows, on line, and on eBay.
I am sure you would like to try on first so at your next quilt show, there will undoubtedly be a fancy thimble dealer. Try on several until you get a good fit and either order one (an expensive gift to yourself) or as if she will write down the size and style you would like and then you have something to go on.
I have found the very old ones come in ore small sizes and i have one that is erfect for long fingernails.
good luck, Mary