Depends on what I'm buying.
Most parts are from the auto parts store but when your looking for OEM parts I go to the dealer. Helps if you have a part #. And a price. Been doing research on this. Best way to find what your Truck was installed with from factory is 1st go here to obtain vehicle code #'s all you need is your VIN number and it will tell you what you have, model code, etc.
http://www.toyodiy.com/
Then I go here for OEM part #'s as it gives you the exact part on your particular truck.
http://toyota-usa.epc-data.com/
Then I go to my dealer website and sub site to parts and punch in part #, it will tell you price and whether or not part is still available to order.
You can also get a printout from the parts counter of what your truck was ordered with and will give the original sticker price, date of production and dealership that ordered it.
This site is a great place to get diagrams of your truck's assembly, just click on whatever diagram you need to bring it up to full screen and save the image or print it. Just make sure it's the diagram for your year. Parts dealer will print it out as well if you ask.
http://www.villagetoyotaparts.com/default.aspx
Now at the parts counter I have always gone to one guy in particular and start up a conversation with him, shoot the bull so to speak, he in return is now giving me discounts and recently some parts for free. Yeah free, I like free! He also owns a Taco and we swap stories so pays to get to know your go to guy in the parts department. I feel confident now that when I want an OEM part he will cut me a deal instead of making me pay full price.