Sagging mattress

Hi all, I was assuming if someone could suggest a good quality mattress in Sydney. Our sagging mattress is terrible. We’ve already changed our mattress three times. These expensive mattress doesn’t seem to last like they used to. The last one started sagging shortly after we got it. It has also developed indentations on each side where we sleep, due to which we wake up with a back pain. We tried to fix it using a foam mattress topper. But, the result was the same. We also tried the mattress, though it helped to smooth out the smaller lumps, it didn’t do a lot to the sagging problem. We tried a temporary fix by putting the two cheap pillows under the indentation. Yeah, it worked but needs to constantly fix the placement of the pillow to have a good night sleep. So, my husband and I’ve decided to purchase a new mattress from a place that one of his friends suggested him.
If anyone has any other suggestion concerning fixing the mattress, please let me know. Maybe I could fix it and put it other use…