A smallish, but well stocked, store on the road to San Kampaeng. Worth a look for a decent selection of rods (not large, but well considered) and a friendly helpful owner. The shop is not flashy, but has everything you need, and advice on hand too – if you speak Thai, or take someone to translate for you.
You can speak to the owner, Somchet Metprasopsun on 053-339-802 or 081-765-8402
See the map of all Chiang Mai fishing shop locations here
View fishingchiangmai.com in a larger map