You can buy minis to represent specific characters from adventure paths, like this one to represent Shalelu Andosana from Rise of the Runelords.

Other than that? An orc is an orc, of course, of course.

The cheapest way to get a bunch of minis is to get the Pathfinder starter box which comes with over 80 cardboard characters and monsters and the stands to use them. (You also get a dry erase, two sided map and some dice!) You can get even more cardboard minis for monsters and such here.

The 3D minis can be expensive. If you don't mind getting random tokens, you can get booster packs for under $20. You'll get 1 large creature and several medium/small creatures, all prepainted. If you want something more specific, you can try ebay or you can make your own at HeroForge. If you really don't mind, you can use anything from coins to rocks to legos.