Or Tor Kor Market has som tam stalls that can make vegan versions without fish sauce or dried shrimp. Reasonable prices for large portions with extra vegetables. They use tamarind and lime for umami instead.

Look for stalls in prepared food sections. Some vendors understand 'jay' (vegan). Also various vegan-adaptable dishes available. Market typically closes around 6pm, accessible via MRT Kamphaeng Phet.