No, you can get OEM parts outside of the dealership.

When I need a part, I usually just google it... turns out the results you turn up are usually dealers, but they sell over the internet, with a parts database, etc. I have had no complaints. One even sent me an awesome Subaru mug for free. :)

I have used SubaruOnlineParts and Subaru-Parts-Dealer successfully. I think they pull from the same parts list (ie, their websites look similar). But they are backed by different physical dealers (and yes, actual dealers), so the prices might be different, blah blah.

So I guess a better answer is yes, and no, you don't have to buy it from a dealership. No, you don't have to go to the dealer, or really interact with one much at all, and part will still be shipped, but yes, technically, it'll probably still be a dealer.

Hope that helps.