3am post, well at least for my time zone, so you must really need a new mattress!
I would say the best deal on a mattress would come from a family member or very close friend who is down sizing or otherwise has a change in their life leading them to get rid of a mattress that is still in great condition. I have two such mattresses in my house that were likely $750-1,250 each new but I paid $250 each for them. Both were less than three years old and were guest room mattresses so saw little use.
Now I have to admit that I sleep on a SleepNumber bed after back surgery a few years ago. Yeah it is pricey but since I need a happy back for decades, I did splurge for sure. I shopped around and waited on a 25% sale but that was still a pretty penny in final price. Now I plan to keep it for 15+ years so I plan to get a good per year cost out of it.