A PVC stand for a cat hammock is built from PVC pipe. PVC is generally used for lawn irrigation and can be put together using a variety of connectors.
It comes in various sizes (e.g., 1/2", 3/4", 1", etc.). The larger size the pipe, the sturdier the stand. 3/4" pipe should be sufficient for most cat hammock projects. The pipe also comes in varying lengths with 8' being common. You'll likely need to cut the pipe to size using a PVC cutter or saw. The connectors fit tightly without using glue (which also allows you take them apart if needed) or you can glue them together if you have a problem with pieces coming apart from use. You can pick up what you need from the lawn irrigation section of Home Depot or Lowes or a similar store.