Hi equestride,
Welcome … and I’m glad you found us.
There is a lot of information on the site but just in case you haven’t read it yet all the most important information, steps, and guidelines that you will need are in the mattress shopping tutorial here which is the first place I would start.
If you follow the steps one by one (adding the optional online step if you are also considering an online purchase) then you will have the best chance of making a great quality and value choice.
Of course you’re always welcome to post any any comments or questions you may have along the way.
With two members of the site looking for a mattress in the same area … some of the stores may end up wondering what’s going on when two people start asking them questions that they aren’t used to answering (or in some cases may not even know how to answer) because as sad as it is … it’s not unusual for those that have spent a couple of hours or so on this site to know more meaningful information about mattresses than most of the mainstream salespeople that sell them 
Phoenix