On ebay, there are a couple for sale. Sadly, they are both in the $1,000-$1,500 range. I am afraid these are the prices you will be finding them for now. The lowest I have seen on ebay for a while was $900, and over 21 people had bid on it. I hope you are able to find a Kermit for a good price. They are such wonderful items to own.
This is the cheapest one I have seen in a little while. It is already out of the box, but still in great condition. You can bid on it for $989.99 or you can buy it now for $1,099.99 on ebay. I would suggest searching "Muppet Replicas" on ebay often if you really want to buy a Kermit. You will never know when one will show up.
Are you looking for the mr photo replica Kermit or a generic Kermit working puppet? There is a person who puts Kermit puppets on eBay that are redesigned plush figures. They are cheap and not bad. They usually sell for between $150 - $200. Other options are to make one yourself. Using the pic of the dissected mr Kermit you can creat a pretty decent replica puppet.