I just go with ikea who happens to sell mattresses without it being their main revenue source. If you want really cheap, DIY foam pads are cheap, but so are water beds.
We have an ikea spring mattress in our spare room - and it's really comfortable. It came rolled up just like a foam mattress does.
I do value good mattresses and IKEA mattresses don’t work for me. My family historically bought mattresses from Costco but not all warehouses carry them.