Best Buy: Sony DualShock 4 Back Button Attachment Black 3004784
Sony's Back Button Attachment is finally back in stock, so here's our review | Ars Technica
Amazon.com: Wireless Xbox Back Button Attachment for Xbox Series S/X Controller, Controller Paddles for Xbox Series S/X/XboxOne/PS4/Switch/PC, Support Turbo/Macro/Audio Jack (Standard Version) Black : Video Games
Best Buy: Sony DualShock 4 Back Button Attachment Black 3004784
ECHTPower PS4 Back Button Attachment Controller Adapter
Sony DUALSHOCK 4 Back Button Attachment for PS4, Black / NEW 711719998006 | eBay
For PS4 Turbo Rear Extension Adapter Controller Paddles Back Button Attachment | eBay