Where to buy 2012 SE Lager parts?
Most bicycle parts are fairly standard, and your bike doesn't look like an exception. There are several mail order stores and local shops that sell bike parts, just pick one. This site is fairly fanatical about not making recommendations, but just knowing your location could help.

Good places to start reading about bike maintenance and repair are http://sheldonbrown.com and http://www.parktool.com/blog/repair-help/.