Do you know where some warrior monk models can be found?
My first stop for 'true' 28mm Sohei would probably be Perry miniatures, although The Assault Group also does some, including some useful archers. But since you're over in the states, you might have more luck with Victory Force or Kingsford miniatures. I'd avoid the Dixons monks for the same reasons as the samurai.
Other than that, it's eBay, I know the Legend of the five rings range had some, and they're probably still in circulation secondhand.
Reaper is my go to.
As far as character models go, they're hard to beat. The only problem is the ever-present one of availability this side of the pond. . . .