I bought my big pothos when it was about 3-4 feet tall/long at my local supermarket. Pothos plants are really easy to grow and sell (plus they are one of the most popular house plants around) so if you check a few stores around you, you can just choose the store with the biggest specimens. As for if parlor palms are good for chams, check this website:
https://flchams.com/chameleon-safe-plant-list/
You probably already knew about it, so I did a little bit of research. I found that they are okay for chams. Here is a link to a similar thread:
https://www.chameleonforums.com/threads/how-safe-is-a-parlor-palm.91672/