I'm currently running a priestess of Sarenrae from Qadira in our campaign, and well... since convincing Hubby GM to solo some RP with me, it has become apparent that a heck of a lot of people react negatively to her native costume. Something to do with the whole slaving issue, which is a big no-no I have learned after our 3 and 1/2 years of making the Stolen Lands a better place. Obviously my character has all the reason in the world to keep her native dress, least of all is her goddess's whole edict to be honest, but secondly... what little IS represented of River Kingdoms costume is typically made of what seems to be cured hides of woodland creatures and low level monsters stitched together with shoe laces.
So... yeah, where can I get some good examples of River Kingdom costume?
Honestly, if you look at the adventure path, the different artists all have different conceptions. One has the aforementioned shoe-lace stitching look, another has what looks to be early 1900s midwestern America, another has 16th century German and a bit of classical Roman.
However, what's relevant is whatever your GM says the dominant style is.