When asking a question, it's not a good idea to then just say "never mind, I've found it/figured it out", as it's more than likely that someone else has the same question. It's better to say "never mind, I've found out that it's like this...".
As mentioned, harvest nettles (and flax/hemp, but you'll have to grow those yourself) using s-a-h or the new (and much appreciated) s-a-i. This can be done somewhat early, before the plant is ready for "real" harvest, but I don't think it can be done in spring. You'd then have to process the ret, dry, and thresh the plants with the Textile craft actions (under the "M" menu, not the "s" one), and then spin the yarn (after making a spindle) from the same menu. Note that both retting and drying take significant time.
As mentioned, a village is the source if you're in a hurry, but there is no guarantee it's present in any particular village, and there may be cultural restrictions as well. Of course, that requires that you have something to trade, as well as sufficient food to go on a village exploration tour.
If I recall correctly, nettles grow along cliffs, water, and in groves (the UrW wiki probably contains somewhat up to date info). I think it may also grow along mountains and on hills. I can't say if it's restricted to the southern part of the world, but wouldn't think so.