How many and how diverse?
Reaper Bones are good.
Wargames factory makes some cheap multipart orks, skeletons and historicals that can easily become bandits or barbarians.
Perry Brothers Medievals will give you the best knights and men at arms you'll ever see.
If you need HOARDS and don't mind monoposes EM4 has the cheapest plastics I've seen. Less than 50 cents an orc. Even after shipping from the
UK it's hard to beat.
http://www.em4miniatures.com/acatalog/Copy_of_Fantasy.html