Buy them straight from Cattex, they're basically always in stock.
Cattex's zeppelin/airships balloons
one is equivalent to the Rifco GL500.
one is equivalent to the Rifco GL700.
one is equivalent to the Rifco GL1200 (Rifco stopped making them so you can't get them anywhere else)
For the GL500 and GL700 I'd recommend buying Rifco ones if possible as they're better quality typically, but for GL1200s you can only get them from Cattex. Also, Cattex ships them with an appropriate clip. Not sure about Rifco.