Well, Khurasan Miniatures make two sets of Myzontid (Lampyroid) Rangers
here and
here that could work, although they are eels rather than serpents.
Also, Khurasan do some
Meso-Nai, which are salamander-inspired rather than snakemen, but which might work for you.
Unfortunately, I'm not sufficiently acquainted with 15mm scale to suggest many more...