Warlord Games had a great little introduction to what can sometimes be hard miniatures to come by- Samurai, Ninja, and Monks. The game Warlords of Erehwon has you covered and coming some Ashigaru Spearmen and Samurai from Pike and Shot (all from Warlord Games) means you can get a very large army for in a great and often harder to get theme.
I say mostly harder because a good set of ninja outside of Titan Forge, are very hard to come by.
Lets take a look at some of the sets for Warlords of Erehwon
You can find these all here at this link and the article with even more info and links
The Warlords of Erehwon sets go for about $40 for 10 miniatures.
