Only large factions field a lot +3 items so far. Factions with at least 5 fiefs, maybe even more like 10. Everyone needs to bring about 15-20 item types for a complete army, so if you have only 1-5 fiefs then you don't have the PP available to spend on +3 (costs 56 PP to bring 1 item to +3), you need to focus on other stuff like trade and then get some basic discounting for your gear but +3 is just too much of a luxury.
If I were you I would message officers of larger factions and ask them if you can buy gear. If they want shoppers coming in, they should be able to provide you a list of where to go. Some might allow it, to make money off taxes, others might not want anyone in their lands.