Its not a common problem with keyboards, although some logitech one's have been known to suffer from it in the past, as have keyboards from most other brands at one time or another.
I'm not aware of any transfers available to put on - they would be no where near as resilient as the 'stickers' that are on the keyboard when supplied. If the original letters are coming off, anything you try and put on will disintegrate in no time at all!
Best bet? Get a new keyboard.