In my current game I have visited essentially every settlement in Driik, Koivula, Sartola, Kiesse, Reemi and many in Kaumo. Of those Driik, Koivulais and Kiesseläis settlements have been revisited once they have had their first major restock (happens after x amount of time, generally several months after you first visited it, rather more muted in Driik, but immediately obvious in Koivula and such when you suddenly see tons of metal armour around where none was before). I have seen maybe half a dozen pairs of regular woolen footrags (I bought two and have two more on my map notes, but I kinda stopped taking notes on that), one par of fine woolen footrags, two pairs of woolen socks at least two pairs of regular linen footrags and a pair of fine line footrags. I have also seen at least two woolen cowls, one masterwork and one fine, but since I found the masterwork early I might have skipped noting one or two regulars.
This matches my previous experience, which includes at least two other games of similar extent and few ones I started just to check on all Driik/Reemi shops. The socks/footrags are there, they may be as rare as masterwork tool axes but you can consistently find them. Cowls are occasionally around. Woolen mittens, never. I think I'll throw another Driik shopping test right away to see what I find.
It's quite possible I'm wrong and I have had unusually unlucky experiences, but at this point I'm kinda begging evidence of someone having a pair of (taken) or (unpaid) woolen mittens in their inventory. If I'm wrong, then take this as suggestion to increase spawn rate of woolen mittens in settlements.