I agree Tropical Fish Shoppe is the best place to get fish and plants in the Fargo area. Really the only place beside Petsmart/Petco. Jeff and Nancy are really nice and helpful.
Another place I have had success with plants is aquariumplants.com. They are located by Sioux Falls SD, and only are a days shipping away which is nice. You can chose ground shipping which is cheaper and it only takes a day.