First, you can do a Google search for mattress for heavy or obese people. One website is
www.absolutecomfortonsale.com which has, they say, mattresses specially designed for people that weigh between 300 and 400 lbs. and also
www.ariamedical.com but those are just two of the many, many links you will find with a simple Google search.
Based on the many complaints that are posted in various sleep forums by heavier folk that have attempted to sleep on "off the shelf" mattress models designed for the masses that don't weigh as much as you I would be reluctant to buy a "regular" mattress as I would suspect that one of those just wouldn't hold up for very long and would quickly lose structure and comfort for you.
My wife and I recently went on a quest of research and testing to try and find a conventional mattress (either innerspring or foam) that would meet our needs (we weigh considerably less than the weight you posted) and after three months of SERIOUS looking we were forced to give up as our testing along with various reviews and user testimonials left us with no obvious choice so we instead bought another waterbed mattress.
I suggest you talk to various waterbed mattress makers re your specific weight issues. We found that modern waterbed mattress engineering delivers a untra-waveless platform with a lot of built-in firmness which is adjustable by adding or removing water until the perfect firmness is found for the individual. We have been sleeping on hard side waterbed mattresses for about 22 years and across that time we have used four mattresses including the one we just purchased. With proper maintenance a waterbed mattress can outlast a conventional mattress easily and there are no springs to wear out nor foam that can have permanent and uncomfortable body indentations over time. We paid only $500 for our most recent mattress which included a new heater and a 3 inch semi-firm memory foam mattress topper - could be a better combo with your weight would be the ultra-ultra firm with a four inch max-firm topper - of course, over the past 22 years as we have gotten older and we learned we needed a bit more firmness and support and, again, modern waterbed mattress technology has evolved to meet our sleep comfort needs far better than traditional and MUCH MORE expensive mattresses plus we both love the gentle heat this type of mattress can have. We are also suffering the consequences of a LOT of money saved - over the past 22 years we have spent about $1200 total for waterbed mattresses including the most recent one.
Final example is my bride has developed several permanent and serious back problems yet she is getting her 8 hours each night without back pain with our waterbed plus topper combination. I am not trying to be a waterbed mattress "commercial" but I woke this morning after - again - eight hours of incredibly comfortable sleep.
Best of Luck