The better options and possibilities I’m aware of in the San Francisco area are listed in post #2 here and in the San Jose area (with some overlap) are listed in post #2 here along with some comments about many of them.
A topper can be a good option if a mattress still has an even sleep surface with no soft spots or sags and just needs some extra softness but it’s only a partial or temporary option at best for a mattress that is sagging because the topper will just follow the sag (see post #4 here).
Again a topper isn’t really a great solution for a saggy mattress (although as you mentioned Costco at least gives you the chance to try it and return it). You can see more of my thoughts about a Costco purchase in post #4 here. The Novaform topper is 3.25 lb density which is low quality memory foam.
Just in case you haven’t read it yet … the tutorial post here has all the basic information, steps, and guidelines that can help you make the best possible mattress choices … and help you avoid the worst ones.