Cate,
To start looking I suggest taking a look at Winnisquammarine.
http://www.winnisquammarine.com/
They are good guys at this place... If you go to there webpage and look at used boats, right now they have a variety of pontoon options.
As others have suggested pontoon log size is important, as well as making sure you have enough power. A small engine is good for putting around, but if the wind kicks up, and your trying to get into a dock or into the boat ramp, you may want a bigger engine with a bigger prop to make sure you can get a good bite, and move the boat around.