Aquarium Center in Randallstown has good selection & prices on dry goods. A few minutes off the beltway. I've also gotten very healthy fish there in the past, but I haven't been there in about a year, and have no experience with their inverts.
There's also Exotic Aquatics, a little further north than Aq. Center, also off the beltway. Highly recommended for livestock, but I can't really speak to their dry goods selection, honestly can't remember. Also, Dr. Mac for frags of course.