I'm against everyone, a "tolerable air mattress" is going be like $80? I would much rather spend a bit more and get something without a doubt much more comfortable and durable. I think this
Should get you by until you could afford something nicer. Its basic, but going to be leaps and bounds better than a air mattress.
I got mine from Amazon. Super affordable and comfy. https://www.amazon.com/dp/B01N0VNL1N?ref=pd_basp_dp_primary