Chair Recommendations please

Hello Alll,

My day job & my side gig requires me to stay on my desk at least 13 hrs on the chair Monday to Friday. I have bought some regular seats but i have started to feel back pain when i'm going to bed. I need to figure this out. What do you guys think i should go for, my budget is $300. I'm okay with facebook market place deals..but i need to know the models to search. Thanks in advance