Hi franklin,
Welcome to the Mattress Forum! 
The first place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that you will need to make the best possible choice … and know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for that are involved in each of them and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability, durability, and value.
The process of looking for a mattress would involve the same steps in the tutorial for someone who is heavier as someone who is lighter but you will likely need firmer materials than average, possibly a thicker mattress (depending on the specifics of the mattress), and you would need to put a greater emphasis on more durable materials as well. Post #3 here also has more information that would be helpful for those with larger or heavier body types.
The tutorial also includes several links to lists of some of the better online options I’m aware of (in the optional online step) and if you let me know your city or zip code I’d be happy to let you know about any of the better local options or possibilities I’m aware of as well.
Phoenix