I don't believe you can reserve them through the internet.
Here is a link for information. http://www.ecatalina.com/transportation_go.cfm
The one on pebbly beach road is where most cruisers do not know about.
You tender into the main pier, most go off to the right from the pier into the main shops and restaurants. Going to the left is not much but the rental place is down the road.
The city of Avalon on Catalina is about one square mile.
Shore excursions are about the same price on your own or through the cruise line.