The internet "store" I use the most is M.B. Klein (modeltrainstuff.com) They also have a retail shop in Maryland which I plan to visit next year when my son moves down to that area for work - need to start saving now lol. Another source I've used is TrainWorld in Brooklyn. I've ordered from them several times several years ago (via phone as they didn't have a website) and was blown away by their superfast shipping. I also visited their store in Brooklyn - way small shop as mail order really is their lifeline, but it was fun as there was an EL right outside (elevated portion of the subway). Unfortunately I don't know if either ships internationally.
If you are taking a road trip that way, the Strasburg Train Shop (near the Strasburg Railroad and the Railroad Museum of Pennsylvania, beside the Choo-Choo Barn) in Strasburg, PA is ok, as is Strasburg Station store in the Rockvale Outlets mall in Lancaster, PA (just north of Strasburg on US30). The Model Railroad Shop in Piscataway, NJ is also worth a visit. The M.B. Klein retail store is pretty good too.