Those will work you can get the fixture on Amazon and then the light bulb as well or you can get the bulb on Pangea https://www.pangeareptile.com/store/arcadia-6-uvb-t5-reptile-bulb.html
I think 18” will work too since it’ll be an inch bigger than the length of but I’m not sure. You’ll probably have to upgrade the enclosure to a bigger one which is why I recommend a 24” which is a 2”
I’ll tag some people who own Jackson’s @JacksJill @Kaizen
Also read through this! All of it! I know it’s long but it’ll help you meanwhile one of them answers 😊
https://chameleonacademy.com/the-jacksons-chameleon-trioceros-jacksonii/
I would say it’s better to get it on pangea because it’s cheaper than some of these options I think a 24” or a 36”would be good. Can you measure the cage preferably the length but all of it would be good too hopefully someone who has a Jackson’s can review your husbandry fully!
Is there an LLLreptile near you they should have some just call ahead. Also petco and petsmart will most likely not have the right one Amazon I think they do let me find the link