There's two (maybe 3) thing to consider when picking a mattress topper: material and thickness. For thickness usually the options are 2" or 3" -- but sometimes there are 1" and 1.5" and 2.5" toppers. The thicker the topper the softer the bed is going to be. For material the three options are typically: memory foam (what you probably have), a high performance polyfoam (like serene foam) or a latex foam. Memory foam is the most common - it's soft and responds to your body slowly. Latex is probably the most expensive -- it's more durable, but tends to feel a bit "firmer" in that it's a super-fast responding foam. A high performance polyfoam will usually be somewhere in-between the two in terms of feel. If you are going with memory foam -- the way to understand the durability is to know the density of the foam. If the company doesn't list or won't tell you a density, it's probably a very low quality foam. A higher quality memory foam will generally be 4.0lbs pcf or higher. Sources for toppers are listed in the DIY mattress guide under the section "sourcing comfort layers." More on reddit.com

Maybe bimart More on reddit.com

Since your existing mattress is sagging in the middle, a topper isn’t going to fill that sag in. BUT, you can put some folded blankets in the saggy valley, then cover with your topper. Another option is to put the topper directly onto the floor and sleep there. One more option is to get a 4x8 plywood sheet (1/4”) to place under the mattress you have, if you currently just have wood slats holding your mattress. But, that’s more money spent, on a maybe solution. More on reddit.com

Do not get a twin xl mattress topper, probably the reason why i started skipping my 9am More on reddit.com