Hi mightynurse,
The first place I would start your research is post #1 here. It has all the basic information, steps, and guidelines you will need to make the best possible choices.
I would start locally and then compare your “best” local choices with what is available to you online and then you can decide which one is the best value for you based on what I call your your personal value equation. You can see some of the pros and cons and risks of buying a mattress online vs locally in this thread.
Dreamfoam is one of the manufacturing members here which means that I consider them to be among the best quality and value available. A forum search on Dreamfoam or on Brooklyn Bedding (you can just click these) will bring up hundreds of posts about them.
If you follow all the steps one by one you will be fine and end up with a great quality/value mattress … regardless of whether you decide to buy locally or online 
When you get to step 3 if you let me know your city or zip I’d be happy to let you know about the better options or possibilities I’m aware of in your area.
Phoenix