Both are at least a little controversial, but so is everything in this hobby (lol)
I'm of the opinion that buying from a chain store is not a crime against nature, but it is ideal to find a local fish store. If googling 'fish stores near me' doesn't yield any results (although I'd be shocked if Baltimore didn't have one), then a chain store works. But the variety won't be as good and the fish may need some extra love. It's great you have a quarantine tank, though, that eliminates a lot of the major concerns in using Petsmart/petco/pet supplies plus/whatever
If you want to order online, I'd recommend AquaHuna. Almost all fish end up getting shipped to pet stores via plane or at least a massive truck, so shipping isn't something your fish will avoid going through either way. Just make sure the company packs the fish well. This will be more expensive because shipping costs $$$ when you have to pay for priority, and express, and the extra heat packs, and whatever else.