I did a search to try and find info on this, but got nothing. Does anyone know of a website or service where I could purchase keyable 3D elements? From time to time, I could use 3D components to include in animated backgrounds. For example science elements like atomic structures, medical devices, ect. Thanks!

Like these ??

Digital Juice: Jump Backs HD 03: Medical/Health, Education & Science (http://www.digitaljuice.com/products/products.asp?pid=32&tab=t3#tabs)

Are you looking for pre-rendered 3D assets? (as in they're are already in some image or video format with an alpha channel)
In this case there's tons of stuff on stock sites like istockphoto.com in their video clips section.

... or are you asking about actual 3D models and such that you can import into Lightwave/Maya/C4D etc and animate and render yourself for your projects?
In that case turbosquid.com is pretty popular.