This thread will give you links to download the CabMaker extension.
This one leads to the Sketchup Extension Warehouse. The other principal source of extensions is SketchUcation Plugin Store. Some authors use only one, some use the other, and a few use both.
Some independent authors have their own website, and don’t use either of the above.
PS. It seems that the EW version is free, but only v5. You still have to register on the cabmaker32.com website - I had previously done that, but too long ago - I’m getting an error message when I try to open a freshly downloaded v5.
The later version(s) are on SketchUcation, and IIRC not free, but have a free trial.