The first 11ac adapters arrived in 2012, with lackluster performance. There are two basic models: thumb drive with internal antennas, and plug in with external, adjustable antenna.
In 2013, 11ac adapters show much better performance and, increasingly, support for 5Gbps USB 3.0. Actual throughput is far below maximum 11ac data rates; sometimes is no better than 11n. Many of the plug-ins work best only with the same vendor’s corresponding consumer-grade access point. So check out reviews.
Here’s an alphabetic sampling of products; prices as of Oct. 10, 2013, based on data from Google Shopping and Amazon.