Yes, there are only a few types of film available now, and some of the older cameras may not work with them. I just ran across these blog posts that explains some more of the technical side of cameras and film cartridges available. [link]
Kodak has recently discontinued Ektachrome 100D but some is still available online. They're introducing a new negative film stock called Vision3 50D: [link]
These film stocks are going to be more expensive but they're another option too: [link]
Check out these guys for affordable transfers to digital: [link]
If you want to get something really cheap, and don't mind taking a small risk, your best bet is usually ebay: [link] If you don't want to take a risk, one of the really big stores, like Adorama, Freestyle, or B&H is a good bet, but it will be more expensive. The last place to look is going to be a store like Frugal Photographer or Amazon, where you have added more unnecessary retail middlemen between the manufacturer and yourself. There are even some knockoff people in China making fakes of some things (not film), although buying that stuff isn't realy ethical and it is a whole lot more risky even than ebay.