If you’re thinking about buying a wireless card to use with DragonFly, Sepherosa Ziehau recommends cards supported by the ath(4), ral(4) and acx(4) drivers. No, I’m not sure which those are.
A running description of activity related to DragonFly BSD.
If you’re after (some measure of) 802.11a support, there was recently a glut of Atheros-based NEC “Aterm” “Warpstar” PA-WL/54AG a/b/g cards in the surplus channel. I can’t say I’ve tested with the BSD ath driver, but they’re supported by madwifi on Linux and it was hard to beat the price.
Netgear’s pre-MIMO 802.11g card is (was?) also Atheros, cheap, and probably performed slightly better (due to higher transmit power) than the NEC. My one encounter with a USR MIMO dongle (Broadcom-based, on Windows) did seem to prove that MIMO hardware blows the previous generation out of the water in an indoor environment, so same might be worth noting if a supported MIMO card actually exists.