I got one with programmable buttons on the bottom at Walmart for like $50. I was just getting a cheap replacement controller and I didn't even know about the extra buttons until I kept accidentally jumping without touching the actual jump button months later.
This one is $84. https://www.walmart.com/ip/PowerA-FUSION-Pro-2-Wired-Controller-for-Xbox-Series-X-S-Black-White/853438652? 

 I've got a second party Switch controller with 2 back panel buttons. I don't know what games I'd use them for though.