You can only get factory Stihl parts from a Stihl dealer, and you can't get those online or mail order. Stihl policy. Sorry. You can find parts new and used on ebay from time to time, and there's a couple of guys on here that have a decent supply of parts. Post over in the Used Equipment forum what you need, and you'r likely to get some help, or at least be pointed in the right direction.
You can find some basic replacement aftermarket parts, like fuel and air filters, bar nuts, sprockets, rims and clutch drums from places like Bailey's and Madsen's. They're not factory Sthil, but they work fine. But for model specific, component level items, you'll need to see your dealer.
Welcome to the site.
Jeff