Best Buy offers the Sony DualShock 4 USB Wireless Adapter for Windows and Mac, model no. 3001625, for $13.99. Opt for in-store pickup to avoid the $3.99 shipping charge. That's the lowest price we could find by $3, outside of the deal below. (For further comparison, we saw it for a buck less last December.)
Of note, with stock severely limited by ZIP code, Micro Center offers it for $9.99 via in-store pickup.