I got mine off of Amazon. I wouldn’t do anything less than 8-12 inches with this one though, unless you have a nice topper you can use.
Best Price Mattress 12 Inch Memory Foam Mattress, Calming Green Tea Infusion, Pressure Relieving, Bed-in-a-Box, CertiPUR-US Certified, Full https://www.amazon.com/dp/B00GSCEJY2/ref=cm_sw_r_cp_api_glt_fabc_H2JW04BWM72296HJT8Q7?psc=1
I had one of these. It is significantly better than Ikea the same price. It's better than my more expensive mattress I have now.
https://www.amazon.com/dp/B00GSCEJY2/ref=twister_B01F8F142E?_encoding=UTF8&psc=1