Zinus Extra Firm iCoil 10 Inch... https://www.amazon.com/dp/B0134FXXKQ?ref=ppx_pop_mob_ap_share

This seemed to work for me, it’s a little cheap but has good firmness. But probably won’t be firm for heavier people. I’m 150lbs and it’s firm for me, my mom and dad slept on it ( ~ 400ish lbs together) and it would sink at the lower back section and give them pain