New Flyer buses in Guatemala

Hi, I'm planning to visit Guatemala to see the New Flyer and maybe some other old American buses (not the school buses). I haven't been able to find much information about which cities still use them other than that Guatemala City itself should have replaced most of them and that their might be some of them still in service at some other cities in the country.

Does anyone have any info on which cities still have a significant number of them in service, which routes they are on or how to buy the tickets?

Thank you for any answers.