When buying stuff I usually check the main retailers, the big online companies, Google Shopping, eBay, Amazon. Sometimes for bigger purchases I super cheap ass it and stack discounts/cashback/cashback programs.
Once I decide on where I'm getting them from I'll check to see if they charge tax and shipping.
Last few times it's been a toss up between eBay and 4WP.
It was eBay for a while. Amazon had some sales or something pop up randomly. Same with 4WP.
Sometimes you can call around and see if you can find a guy that will sell tires for cost and just be interested in collecting for labor. There's a bunch of guys around here that do that. They have some tire network where they can source out tires and get them delivered to them. It's like when batch numbers get older and have been on the shelf, they start to discount them heavily.