The sensor strip that came in the Kit of Parts is documented
here. You can't buy more of those, but you might be able to convince a friendly team to let you use any that they didn't.
There are other sources for similar components, but I don't have details since we didn't buy or use them.
We use the US Digital quadrature encoders included with the AndyMark SuperShifter gearbox. They work perfectly with Kevin Watson's encoder support library for FRC. I'm the one who specified those parts for our team's use a year ago, so I do know where to get them and how to connect them. All that information is here:
http://www.usdigital.com/products/e4p/