I have the manual...but that is a wee-bit cumbersome in the cockpit!
I looked as well, they have only the manual. Since they do have the quick reference cards for every other item I've wanted, I'm thinking they may not have created one for the 155.Have you tried the Garmin Website?
I know I posted this before - Garmin 155XL TSO Quick Reference Card
GPS 155XL TSO quick reference guide
Part Number: 190-00067-21
Suggested Retail Price: $ 15.00 USD