Hi ewlang,
I would start with the tutorial post here which has the basic information, steps, and guidelines that can help you make the best choices … and know how to avoid the worst ones.
I would keep in mind that every layer of a mattress (or mattress/topper combination) can affect every other layer so your mattress with a 3" memory foam topper may not feel the same as another mattress with a 3" layer of a different type or density of memory foam on top or where the layers underneath it are different from your mattress.
The tutorial post also has a link to a list of some of the better online memory foam mattress retailers and manufacturers I’m aware of and many of these use widely available memory foam mattresses as a reference point for firmness/softness (often Tempurpedic) so you can test a local mattress to get a better sense of what they feel like.
If you let me know your city or zip code I’d be happy to let you know of any of the better options or possibilities I’m aware of that are close to you.
Phoenix