As far as finding an animal like this there are sadly no guarantees you'll eliminate every possible place. I remember an interesting story about a ferret that ended up inside of a box that was returned at Walmart!
Start with any place that rents tools or plumbing equipment nearby, do some searches on google for camera plumbing snake rental, tool rentals, if you still come up short call some local plumbers and ask them if they have one or knows of anyone that might that you could rent or use. Explain the situation. Maybe you can get a good deal.
You might be able to use a regular plumbing snake and attach a small camera and light to it. (Yes, you risk damaging the camera or other things) You could set the camera to record a video and then see it turns out. Be careful not to damage anything inside the ducts or the fan. That's probably what I would do if it were me but I'm also pretty handy with kludging things.
Disclaimer: I don't know what might all could go wrong and if you mess it up don't blame me!