Any ath9k-htc model should work with free firmware. You can try to find
wikidevi.com on web archive. With the site down I can no longer search for
devices based on ath9k-htc.
Beware that the USB WLAN adapters bearing the same commodity name but with
different hardware revisions are different hardware. And their chipset can
vary a lot.
And don't ask for "free/libre drivers" any more. Practically almost every
WLAN card has free/libre drivers. But most of them don't work without
non-free firmware. Keep in mind that driver and firmware are different.
(Once someone else asked me for a WLAN card with free driver. I gave him a
Realtek one. He asked me why it didn't work on a free distribution. I replied
"you asked for a card that had free driver, not a card that didn't require
non-free firmware to work".)