I think I have just about figured out the rhyme and reason of the various furniture merchants. So here goes...
-Each zone seems to have a general home goods merchant
-Each zone seems to have an achievement furnisher with furniture unique to the race represented in that zone/the questline which took place there
-Each zone's general home furnisher also seems to sell foliage and garden items which are representative of the climate of the zone in which the furnisher is
-Each alliance capital seems to have the same achievement furnisher who's furniture is gated by completion of the main quest
-Cyrodiil has a furnisher who sells furniture which is unlocked through PvP, naturally
I don't know if this is a comprehensive list, and I don't know if all of what I just said is super obvious and everyone already knew about it, but as someone who wandered around for a while trying to find out where to buy what furniture, I hope that this list will help someone sharing my struggle.
(If I've missed anything, or if there is already a resource available for locating a specific piece of furniture, please let me know! Thanks!)