Hardware/Drygoods - anywhere - I look for best price. Online for pumps, lighting, RO/DI unit and additivies. This site for many test kits.
Live Rock - I've hand pick most all my rock from one of the coolest saltwater coral and fish places on the planet. I would maybe consider a large LR order online - but would first order a small sample from them to see it's overall quality.
Fish - I've purchased all my fish ( except 2 little False Percs ) from one guy here in town - never had a case of ich or disease from him.
He runs SANO in all his systems - and has some of the nicest fish I've seen at resonable prices, and offers 50% store credit on any healthy fish returns for ANY reason.
Corals - mostly from the same guy here in town, although I did make (1) online purchase. Was not completely satisfied with the shipment. They sent me a "doomed" flower pot instead of what I ordered, I hesitated to keep it, but did, and it slowly withered away.
Clean up crews - THIS site for the most part. Can't beat the price and I've only lost some snails due to some agressive blue legs. They've been banished to the refugium.