...what are you looking for: the 3-D opal glass letters that fit into the face of a sign, or the ones used in early movie marquees?
...either way, you might be able to have some made...regarding the formed, opal glass letters, that seems to be what most people undertaking a restoration of those signs have done...there was an short article about Ralph White's Chevrolet sign in PCM, I think...he had new ceramic slump molds created from the 'windows' in the sign, and had new letters made...
...yes, it's every bit as expensive as it sounds...
...I've got a round, two-sided 'That Good Gulf Gasoline' opal glass sign that is in surprisingly good shape (at least I think so, given it's fragile construction)...I contacted Flexlume (the maker of the sign - they're STILL around!), and sent tracings of a couple of the 'windows' where the letters would fit, but they didn't have those molds...depending on what you are looking for, they might have them...
...you might try Leo Mathieu, I believe he had some opal glass letters made...
...good luck!