Quick question.

Is there a way to switch between multiple devices to feed into one line input on a mixer? I.e. I have a lot of sound sources but they don't necessarily run at the same time, and unplugging and plugging in on stage is obviously not feasible. Does a small line input switcher exist? Does this make sense?

yup look into A/B/Y pedals and switches.

This, and they do exist in stereo, but if they're all the same device or at least have the same output level/impedance, you can get or build a very simple summing mixer with no level controls that should work fine. I did this with a band for awhile, just a little box with connectors and resistors - had three gameboys plugged in just to switch songs quickly but only used one at a time.

could go for one of these, just ignore the composite video jacks entirely .. only $4
http://www.ebay.com/itm/like/271528787996

Thanks guys! Perfect. This helps me out a lot.