Yah, seaworld is great. Their SW stuff is better than the FW, but they have a lot of supplies and sometimes less-common fish. Usually quite healthy, too

I just bought 24 glowlights there on Saturday.
There's also a Petsmart, Petco, and some other store right in town (being Salem). Chains, but better than average.
The Petsmart had some Bamboo shrimp last week. Seaworld usually only has ghost shrimp, they were out last week. I think the chain stores also had some snails, run of the mill mystery snails.
The Petco/PetQuarters in newington usually don't carry any shrimp, though last week the Petco in Newington also had some huge Bamboo shrimp. Petquarters had some ghost shrimp (it's all the carry). Both had mystery snails and maybe a couple ramshorns thrown in.
If you also are into SW, Little Shop of Pets in downtown Portsmouth has a lot of stuff. Great corals, too.