Except for one stall at the back of Khamtieng market, I haven't found a good source of vegetable plants to stick in a garden.

peppers, tomatoes, mint, etc

I haven't personally looked for the veggy types there but how about that huge plant market directly behind the north (Mae Rim?) Tesco Lotus?

The Khamtieng market is the big one behind Tesco. It's all flowers and trees. A great place to visit, but no vegetables.

Just talking to the wife about this and she believes " Puan-Ka-Sed " plant shop has fruit and vegetable plants. Ph#: 086-917-9759 (Thai speaking only, need translator). From Chiang Mai go south on the ChiangMai - Hong Dong hwy. past the Hong Dong Big C intersection to the next big superhighway intersection. Turn right there (west) and follow to near the end where you'll see a wood shop on the left with wood doors displayed out front. Puan-Ka-Sed is right after this, on the left also.

Many of those are usually/easily grown from seed.