The word "bicycle" makes me think of old times when Raleigh bicycles were king, and below is a photo of one of those old classic Raleigh bicycle:
Photo of Raleigh bicycle is property of Rowan McLaughlin
During those times, multi-gear bicycle were very rare, but now you can hardly find any modern bicycle without at least 3 gears. Speaking of gears, the most famous brand is Shimano.
Now Shopwiki, the Wiki of online shopping which includes, unlike other websites, not only paid insertions, but also catalogue everything they can find, also have a section for specialized bicycles which include mountain bikes.