I am trying to pick out some motors for my robot project and I need to be able to mount shaft encoders. Do I need to find motors with a back shaft or can any motor have a shaft encoder? If I need motors with back shafts, where can I find those? Or where do I find shaft encoders that mount on any motor? And how do I mount them? Thanks in advance!
Encoders can be mounted anywhere in the drive train depending on your application though they are most effective mounted on the motor shaft. Here is a selection of appropriate motors with and without encoders: http://www.robotshop.com/planetary-gearmotors.html. A little googling will turn up others.
Do I need to find motors with a back shaft or can any motor have a shaft encoder?
It depends on your encoder, some have just a single connection, in which case you need a back shaft on your motor. Others are through shaft and so your motor doesn't have to have a back shaft. You can also couple it with gears so you have a lot of choice.