Baltimore:
Mid Atlantic Store fixtures (website)
(new and used?)
North Brentwood:
Fixtures Plus (website)
(new and used)
Rockville:
Dusty's Depot (website)
(used)
Nearest out of state dealers in Philadelphia and Richmond.The Maryland and Washington D.C. area shipping costs will be high per mile than many regions, but dealers to the North of South are close enough to be good options.
The dealers you do have in Maryland seem very solid. If they don't have what you need in stock, you probably are asking for odd sized retail shelving.
