If you can sew or have a friend who can, you can also make a "topper". 1) Measure the length and width of the depression on your mattress. 2) calculate how much fabric you need to make your topper cover (an old queen or king size flat sheet can be used for this, depending on the size of the depression on your mattress). Basically, you will be making an envelope that is only about 4 to 6 inches longer and wider than the depression in your mattress.
3) Once you made the envelope with fabric, you can stuff it with pillow stuffing which you can buy or if you have old pillows around pop them open and get the stuffing for you topper. You must stuff it very taught but shallow . 4) Set your topper on the bed under the fitted sheet (the one with the elastic on the corners) making sure its centered over the depression. 5) Now it's ready to use.
If you are really crafty, you could make the topper fit the curvatures of your body regardless of your usual sleep position. Just have someone measure the areas where there's space between your body and the topper. I'm a back sleeper and doing this has worked for me. Saved me from having to buy a new mattress. Good luck!