According to Wikipedia, all Master System Cartridges differ in style in every Region, so they won't fit in another Region Console.
After looking up "buy retro games online usa" on Google, I've found quite a few sites:
http://www.tradengames.com/ProductCart/pc/viewCategories.asp?idCategory=100
http://www.collectorscardsandgames.com/new/SegaMasterSystem.html
http://www.estarland.com/index.asp?page=SegaMasterSystem&category=Games
Source: http://www.racketboy.com/forum/viewtopic.php?t=25415