When it comes to Liverock, I would advise you to go mail order, its cheaper and better. The two major Liverock dealers that I think are the absolute best are www.paragonsales.com and www.premiumaquatics.com
Premium Aquatics is a great place to do business with dry goods also, great prices, great service, honest people.
As for the best place in MN. The best place in my opinion is Something Fishy, good stuff at OK prices. Another place in the same area (about 3 blocks from SF) is World of fish. I personally HATE WOF, they are rude arrogant people, their prices are terrible, but they do have decent stock. So my advice is to go to WOF, find something that you like, then go to Something fishy and buy it from them. There are a few others, but I haven't been to a lot of them. Forest lake pets is ok, there is a decent one behind Cub Foods in Mankato, Wet World is ok. I would ask this ??? in the TCMAS forum, you'll get a LOT more answers.