Petzl Zipline Pulleys
Petzl makes a couple reasonably priced options for backyard zipline pulleys. These are very popular pulleys and are easily disconnected from the zipline cable.
Petzl Tandem Speed (Transport Pulley)
If you click on the image and read reviews for this trolley and other Petzl transport pulleys, please pay special attention to some of the negative comments. This line of pulley is not the best choice for ziplining. It will run safely, but many of the cheaper ones were designed for use on rope to transport gear, and so the sheaves will wear out when riding on cable. However, for occasional use, this is about the lowest end pulley I would buy for a zipline.
Petzl Trac Double Pulley
This pulley has some great advantages. The most obvious is the built in carabiner. This makes it easy to unhook the pulley and take it with you, which is attractive to zipline tour operators because it saves time and helps send more participants through the course in a day. It is also very helpful on canopy tours where you have to disconnect from one cable and then connect to the next zipline. While some commercial operators have complained that this pulley lacks durability, they are relatively cheap compared to the alternative, and they are plenty durable enough for backyard ziplines. This pulley is best for ziplines where participants are wearing harnesses and/or rides where the pulley is being disconnected and carried back to the launching point.