Best place to buy original PS3 controllers?

If you have a PS4, use one of those controllers. Some buttons won`t work unless plugged up though.

Ebay is great for older gaming stuff.

In general try to avoid resellers who run shops that sell tons of stuff, that's where you're likely to get fake merchandise. Not always of course, not even usually, just depends. Check the actual reviews/feedback and see if there are negative ones and if so what they say.

Try to find regular people selling their personal older stuff if you are worried about authenticity.

It's pretty easy to tell the difference between the two type of sellers.