the title says it all, does anyone know where I can order a wagon, horses, oxen, etc at the correct scale?
There are tons of these available online from respected miniatures companies. Try Foundry:
Great stuff on that foundry page. I bet most historical miniatures companies have similar offerings.
I made some of my own, using cardstock and painting them to look like boards. Had to make solid wheels, though, using metal washers, because spoked wheels are a b'tch to make!
I have painted stuff from Reaper and Games Workshop. I can't speak to those made by other companies. Some of it is kinda expensive and time-consuming to paint, especially if it will not see much time on the battle mat. But its nice having the pieces around... so I can dust it off every 2-3 years and use it... :)
Actually, if you're keen on wilderness adventures, more than on dungeon adventures, you have plenty of opportunities to have wagons and caravans and such. Think Jade Regent. If not the players' own, then saving an errant merchant or peasant is always a good trope. He might even reward you with the biggest turnip every seen by man nor boy!
Tabletop Props recently completed a Kickstarter for pre-painted wagons. They should appear in retail outlets later this year.
https://www.kickstarter.com/projects/1750773027/tabletop-props-covered-wago n