They're actually part of window screen hardware. They're intended to hold the screen into a door, window casing, etc. At home centers you can usually find them where they sell door and window hardware - eg. handles, catches, screen fasteners, etc. I've used them only on one type of cage that I made. I greatly prefer to use casement-type catches where the construction is robust enough to support them. I like the postive locking feeling the casement catch gives me, also at a glance I can be sure the lock is fully engaged.
This the plastic screen window catch as I installed on a plastic drawer box type cage...
http://img.photobucket.com/albums/v483/MCampbell/plasticscreenlatch.jpg
...and this is the casement latch as installed on one of my latest caging experiments.
http://img.photobucket.com/albums/v483/MCampbell/casementlatchwood.jpg
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Matt Campbell
25 years herp keeping experience
Full-time zookeeper
Personal collection - 21 snakes (9 genera), 20 lizards (4 genera), 6 chelonians (2 genera)