I've done the same and I have to say: Most CK monsters can easily proxied with the base set. You have dragons, you have 4 2x1 hounds, ogre like creatures (ettins) and a bunch of other 2x2 monsters you can use very well for monsters this size.
On board game geek there are even nice looking 3d tokens for all monsters (use it to give you at least a reference of how big the monsters are).
That being said, you only really need to get tokens for the kobolds (where you need up to 12 white and 3 red monsters of the same kind).
If you really want genuine FFG miniatures for your favorite monsters you can purchase the Monster &Hero packs if they come with them.
Otherwise I found reaperminiatures to produce nice looking minis for a reasonable price (2-3$ per miniature) and I am currently buying some of my favorite monsters there that aren't released by FFG.