For o-rings and the like you have a few options. You have palmer's pursuit (google and you'll find their website), short304, or MaD - both are on this forum. I love short304, excellent guy to deal with, but in this case I'd recommend MaD as his seal kit has EVERYTHING needed to get her running at the best price. Plus he's an excellent fellow. Search him in the member directory an shoot him a PM.
If you're looking to use 12 grams and want the best deal on a lot of them search Titus - also an awesome dude.
The more knowledgeable guys will answer your other questions. Hopefully this helps.