Hi pixbroker,
I think your two best bets for latex in the area would be the first two links in the Las Vegas list.
R & S matress carries a line of Brooklyn Bedding latex mattresses and they may be able to build a custom size for you and Ergobedroom also has a variety of latex mattresses that would be good for testing and they may also be able to order a custom size.
Post #2 here has more about the differences between latex and memory foam but they are not really comparable because memory foam changes its softness and response with temperature, humidity, and time while latex just responds to pressure and is more consistent with different conditions. The best way to know what latex “feels” like either by itself or with memory foam on top of it is to test it in person.
The thickness of the latex mattress will be less important than the thickness and firmness of each of the layers and the overall design of the mattress (including the cover) because regardless of the thickness of the mattress, the design will have more to do with how it feels and performs although in very general terms (all other things being equal) thicker mattresses tend to feel softer. A “typical” size for a latex mattress would be in the range of 8" to 9" which has room for a firmer support layer and a comfort layer on top but both thinner and thicker can also work well depending on the person and the design.
It would be great if one of them had a memory foam topper in their store that you could add on top of any latex mattresses you were trying so you could test something closer to the combination you are considering. Without this I would probably suggest choosing one that was a little firmer than you would otherwise be comfortable with to compensate for the fact that you would be adding a softer memory foam topper. You may also be fine with a basic 6" mattress that used the memory foam layer on top of it to replace any softer latex that would otherwise be part of the comfort layers.
Phoenix