This will be a rather long way of answering your question.
The absolute best source for A:M models is right here in the forum.
There are enough artists and modelers in the community that just about anything can be modeled (if it hasn't been modeled before).
There are two aspects of model distribution to consider. I'm not sure which you are interesting so I'll assume both:
Persuing the first means of obtaining models will generally (but certainly not always!) take more time to identify, locate, obtain and modify (assuming its not exactly to your specifications already). The quality will generally be lower.
The second means is more of a solid approach but can become quite expensive. A realistically modeled and rigged model takes time to build. The quality is almost always higher.
So, the absolute best source of models for A:M is right here just make your wishes known.
There are other sources of models available too:
There is http://www.3dfrogstore.com/
(They do custom orders too)
There is the Extra CD (Approx. 500MB of data with quite a few models. From Hash Inc)
There is the Extra DVD (Approx. 5GB of data with a whole lot of models. From Hash Inc)
A lot of A:M Users also maintain websites with models too.
Is there anything in particular you are looking for?