Nakhon Phanom doesn't seem to be a big shopping town. Although there are some villages in the province that are famous for their silk, you won't find much of it on sale. There are no big souvenir shops or stalls around town.
Indochine Market is a small market facing the river just a short way north of the old car ferry ramp. The first floor is full of small shops selling just about anything you can name, although it's all mostly cheap stuff. There are a few stalls selling locally produced dried fruits and other delicacies. Upstairs is a sort of food court.
The city's main market is a few blocks away from the river. There is a large covered area that's active in the daytime, and a nearby street that turns into a food market at night.