I don’t label the cages any more, and these aren’t exactly what I used, but close. The ones I had were for use with business cards, but it looks like business cards would fit in these, too.
https://www.amazon.com/Clear-Plastic...g+holder&psc=1
I used double sided tape to mount the tag holders to the cage, then I could just switch out the cards as needed. I used sheets of printable business cards so that anyone could read it rather than trying to decipher my writing.