I always get my
seeds from a variety of sources - local stores, online companies, and many from my personal stock that I save from year to year. One of my favorite seed sources is Fedco - a co-op from Maine; their seeds are relatively cheaply priced because they don't spend ungodly amounts of money on advertising.
Regarding what fintuckyfarms said about tomatoes, I think just about any cultivar will thrive hydroponically. I myself grow both determinate (bush) and indeterminate (vining) varieties outdoors, but I stick with determinate indoors because they are more manageable. Here is a list of some of the popular (commercial) greenhouse varieties used for hydroponic production:
Beefsteak: Trust, Quest, Match, Blitz, Geronimo
Cherry: Favorita, Conchita
Cocktail: Picolino (red), Flavorino (plum)
Truss (Cluster): Locarno (yellow), DRK 902 (orange), Tradiro (red), Ambiance (red)
Seldom are these varieties offered in mainstream seed catalogs, for good reason - they can be VERY expensive. I personally stick with a nice mix of heirloom and cheaper hybrid seeds.