I shop for the best price. When I settle on a particular product, I usually search on Google by name or part number to see where I can find it for cheapest. I also check Amazon (no, not Amazon Marketplace), which often has the best price on items you wouldn't expect them to have. Most of it usually ships free.
If it's something I don't mind buying used, I check the member classifieds here, although a lot of people here seem to think their used parts are worth as much as new, and the truly good deals seem to go very quickly. So whether you score the deal is a large part luck. I also check eBay. I will sometimes use SearchTempest.com to search Craigslist, just in case.
For my Jeep, I have a lot of luck finding good deals on parts using
Frugal Mechanic, which is a parts search engine that points you to other sites. I've only found a couple of items for the BRZ that way, but that may change as mainstream parts suppliers pick up more parts for the twins. The good thing about FM is that it will often include discount codes.
I find that sponsors of this forum generally do NOT have the best prices or shipping terms. I have purchased a few items from forum vendors, but in those cases it was always because they were the only vendors selling those particular items. I haven't had any problems with forum vendors, but I really think people here are willing to pay way too much vendors out of some kind of irrational religious loyalty. Several times I have checked the price from a forum sponsor shouting that they had the absolutely lowest prices anywhere, only to find it for 20% less from another vendor or on Amazon and with free shipping. I really don't understand it, because the Jeep vendors I deal with for my other vehicle usually give really good discounts to Jeep forum members.