Perhaps you should read some of the other posts again. There are no retailers. There's only 1 way to get quality airsoft guns. Get age verified and you'll have access to the classifieds and retailer sections on this forum. Outside of that, all you'll find are lower end clear guns. The only way you'll get to see them in person is if you go to a game and ask people to see their guns.
And FPS means nothing for guns. It's all about the quality of the materials, both internal and external. Some brands are known for better internals, some externals, some have a decent balance of both.
As for how they work, there are 3 types of airsoft guns - gas, spring, and electric.
Most pistols are gas, and run on either propane (aka Green Gas) or HFC 134a (computer duster). Some will use CO2. There are also spring pistols, but those generally don't get used much as it required cocking with every shot. There are also electric pistols, but they tend to be rather low power. But the nice thing is that they work in cold weather, where propane and duster don't work.
For most assault rifles, the system is electric. Sniper rifles tend to be either spring / bolt action, or gas powered, although there are some electric ones too.