There are some cheap memory foam mattresses on Amazon if you want to buy something new, can also go on Craigslist or a used furniture store to see if you can find some used one you like. Used price drops by a lot since no one wants to buy them, you can easily get something that was 1200+ dollars new for around your budget. I really don't get the hate for used mattresses, whenever you rent a furnished apartment or stay in a hotel it comes with a mattress anyway, so it's quite normal to sleep on a used one as long as its clean. And then you have waterproof protectors which everyone here suggests using anyway, so you're not even in contact with it in any shape or form if you want.