The outdoor spa at my house was already installed when I bought the house. There was no documentation and it was in very serious need of a good cleaning. Not knowing the brand or what filters it used I drove around to a dozen so-called spa stores. None of them carried supplies for any other models than what they sold.
After some online research I found a place that sold parts at wholesale prices for virtually every brand and model spa ever made and (was I in luck) their distribution center was located in Phoenix.
Since then I have replaced both the heater and the control unit in addition to a new cover (all relatively expensive parts) and I have bought all of my filters and chemicals from Spaworks. I checked their prices against the famous online retailer that sells "everything" and the prices are within pennies of each other.
I like to support places in my community, and I like the fact that I see the same employees every time I visit. I don't have to remember my filter number, I just walk in and they pull up my details and always have the part I need in stock. It's also a plus that the staff is friendly and seem to know every possible thing about spas.