I purchased some apparently fake oem parts recently on internet… caused all kinds of elusive issues on my Prius… upstream o2 sensor. See YouTube video
Buy parts from reputable sources only. Never heard of anybody getting a fake directly from a Toyota dealer. Many locations the price is considerably lower if it is ordered through their web site. Shop around, dealer prices vary on the exact same item. Sadly Toyota's own parts site removed the ability to do this easily, so now one has to go to each site individually or some will be missed. Never heard of a fake from RockAuto. Amazon, EBay? Assume new ones are fakes. Not sure about used parts. Given the level of larceny on these sites I can imagine some neer do well dirtying up a fake and selling it as a used OEM.
I've looked stuff up on Amayama, which has a decent interface, then plug the part no's in at Toyota USA's gong show.
Right, Toyota broke their site in (at least) two ways. The first was that it became more difficult to find a part by description. Looking them up on Amayama works around that. I sometimes use RockAuto the same way, the aftermarket parts usually list the OEM part number to which they correspond. The second was that they used to have links to all the local dealers from the page for a given part, so one could quickly click through those to find the local online prices. Now, while it still lists some dealers, many are no longer accessible in that manner. My guess is that Toyota now charges for this service, and some dealers didn't want to pay. So to check everybody locally requires a lot more work, entering the same values into each dealer's web site.