The only thing that matters is the chipset... which is usually not even
specified on the wrapping. Worse: the manufacturer may change the chipset on
a same model, which may used to work with free or no firmware but not anymore
after the change.
As far as I know, The only reliable way to buy a new card without testing it
and to be sure it will work with free or no firmware is to buy from
http://libre.thinkpenguin.com or http://tehnoetic.com (both ship
internationally).
You may buy second-hand if the vendor indicates the chipset. The Atheros
chipsets AR7010, AR9170, AR9270 and AR9271 (maybe others but not AR10k) are
known to work. Some Realtek chipsets recently started to work too. For
instance, my previous computer had a RTL8188EE chipset that started to work
with the newer kernel in Trisquel 8:
https://trisquel.info/forum/trisquel-8-codename-flidas-release-candidate-ready-testing?page=1#comment-130659
