Hi rose24,
Your budget is certainly enough to purchase a high quality latex mattress.
The first place I would start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that you will need to make the best possible choices. It also includes a link to a list of the members here that sell mattresses online and it includes many that sell latex mattresses with a wide range of designs, options, and features inside your budget range.
Post #6 here also includes more information about the different types and blends of latex so you are comfortable with limiting your choices to 100% natural latex.
There may also be some good options available that are local to you and if you let me know your city or zip code I’d be happy to let you know of any of the better local options or possibilities I’m aware of.
Phoenix