The best PS4 controllers out there feature programmable back paddles, and earlier this year, Sony released an attachment that equipped its DualShock 4 with them. That move proved popular, as the back button attachment sold out quite quickly. Sony announced last week that more would be back in stock in the US this week, and that looks to be have held true. The DualShock 4 back button attachment costs $30 and is currently available at some major retailers. Some have it in stock, while others are offering backorders that let you order ahead of time and receive it when more stock becomes available.
Right now, you can get one shipped to you from Best Buy and Walmart for its standard $30 price tag. You can also order one from GameStop via backorder. The peripheral has proven to be quite popular, however, so this current round of stock will likely sell out soon. You can also get it from PlayStation Direct.
DualShock 4 back button attachment