Sarin said:
Horseman javelins are only from loot, right? Or is there another way to get them? And what's the best way to get them from loot...
The best way to get them it to admit to yourself that the idea that bags of javelins produced by the hundred for horseman across the British isles shouldn't be ultra rare in the first place, and just allow yourself to buy them from the cheat menu. Absent that, the best way to get them is to fight the troops that carry them (you can check what troops carry what weapons in the troop tree screen, accessible via the Camp menu) and build up a morale bonus high enough to allow you to demand first ownership of the loot (expect a 42 point drop in morale if you do, plan accordingly). Large groups of british horseman sometimes appear as Masterless Men around the Briton kingdoms (Gwynedd, Glywyssing, Alt Cult, Brycheiniog), which would give the best chance I know of to nab some of them.