Usually, the parts counter at the local dealership... you're guaranteed to get the right part in one visit... but in some cases it pays to shop around. Example: a new OEM wheel for my Prius was over $400, but after shopping around I found a reconditioned used wheel (which looked like new) for only $150. Yeah, you can save a few bucks with generic gaskets and filters, but the piece of mind and less hassle of OEM is worth the difference to me. After all, there isn't much to replace for the first 100k miles!
I only buy from the dealer if I have to. I know there are those that say buy only from the dealer but, I'm so cheap, I can't force myself to do that. I usually buy at Advance Auto or Autozone.
There are a few Toyota dealers that sell parts at ~25% discount from list for web orders. I buy my Toyota parts from Champion Toyota, Houston, which is one such dealer. Since you have to pay a shipment charge, it makes sense to think about your future requirements and purchase in bulk. For Toyota-branded fluids or the 12V battery, I would buy from my local Toyota dealer. Those items are either too expensive to ship or prohibited for shipping.
