Well for some of the more common plants such as Java fern, Hornwort and such you might just want to go to Petsmart or Petco. They're about 3.00 dollars there. For the more uncommon stuff I say World of fish. As for fishes it depends on what you want. Aqualand tends to have cheaper fish; World of fish more variety, but a little more expensive. You also might want to try out Petland and Twin city reptiles.
As for the best fish store in Twincity, I thnk World of fish tends to be it since the have a wide variety of fish and plants. If your looking to save money its better to shop around and check out the chains stores.