The three places I frequent the most often for parts are: MidwayUSA dot com, Brownells dot com and gunpartscorp dot com [Numrich]. After that I go to the manufacturers.
I honestly don't remember exactly whom I got the parts from, but I am pretty sure it was Marlin.
You are right. Taking the entire mechanism apart was a real PAIN to fix everything. Before you go to that step you should frequent rimfirecentral and the marlinowners website to pick up tips on parts that could be giving you fits. That way you can order everything you need at once and only pull it apart once.
I asked about storing 'cocked' as I did that and couldn't figure out the reason I was getting light strikes on the rifle years later. DOH!
The key with these rifles is a good bore. Outside looks be darned, the mechanism/bore is what makes it accurate!