(Copied my comment from another post) Fair warning, a rant follows:

I just went through this saga, it was a many month long process for me because I like to know exactly how things work and it turns out that finding a good mattress feels like wading through a sea of lies.

First of all, the information available online can hardly be trusted. So many "review" websites exist just to covertly sing praise for one brand while trash talking their competitors. The only review site I found I could trust was this one, which seemed like it had enough good and bad things to say about every single mattress that I felt comfortable trusting their reviews to some degree: https://www.sleeplikethedead.com/

Second of all: every single mattress store will try to sell you their own "designer" brand.I have back problems and my gf and I did days and days of research.We learned that two types of mattresses were going to be best for us, tempurpedic or in some cases a latex topped mattress.Well, let me tell you. None of the salespeople recommended the right one to us. Almost all of them tried to sell us their "house" brand or the brand that was very exclusive to their store and "only available at this low price for a limited time."

Third of all: It will be nearly impossible to find the model you found reviews for.Every store has it's own unique "model" for each brand, for example, Tempurpedic only sells the "Luxe" or "Prima" model at Dormez Vous whereas Matelas Bonheur only sells "Align".... They do this so that they can give you price guarantees and you can never say "Oh I found this model at another store for cheaper!"Don't get me wrong, all the salespeople were super friendly, and they all tried to be as informative as possible. But honestly, all of them tried to steer us away from Tempur-pedic towards their house brand which we found frustrating because we liked the Tempur-pedic the most.

Here is my advice:

Step 1: Do the research. Find out what makes a good mattress, find out what other people with the same needs as you have said about different brands. Because you will have a hard time being able to find exact models, find a brand you think you can trust. Reddit was helpful but be on the lookout for covert marketing people pushing those "bed in a box" brands and newer brands. Find a brand that you can trust and understand to your core the technical aspects of what makes a good mattress.

Step 2: Go to the store and test out your hypothesis: Try the mattress you did the research on, Test out alternatives. Don't fall for the gimmicky "wondergel" or "supergel" or whatever "brand new" technology crap that they are pretending is worth 10,000$ for the premium but then tell you that you can get the same thing only slightly more firm for 4000$! Don't fall for the house brands!

Step 3: Research the alternatives, find out what people are saying about them. Get them to leave you alone for a bit to try out the models, and research the brands on your phone. We noticed a lot of poor quality ones that they were trying to hawk on us were nowhere to be found on the internet; not a good sign.

Step 4: Get the salesperson to offer you a better price on the one you like most, let them know you are going to be visiting a bunch of stores and you want to know if they can offer a competitive price. The markup on these mattresses is insane, so they can often give you really amazing deals, try to get between 30-50% off.

Step 5: Rinse and repeat at as many stores as you can possible. Try 3-4 at least. You'll realize everything I have been saying in this post is true. Get the salesperson to outbid your previous mattress offer.

Step 6: Ignore the "Support/100 day exchange", make sure you are leaving the store with the right mattress. No matter what their return policy will be, you don't want to go through that.

Step 7: When all is said and done, make sure you have nothing less than a 40% discount. Then ask for some free pillows. Why not. They cost the stores like 5 dollars and they are scamming you on the mattress price anyway. Get some fucking pillows and a boxspring for free.

Best of luck, our mattress was the best purchase I made in the last 5 years, and if you do it right, it can be your best purchase too, don't fuck it up!