Ok. I will tell u step, by step.
1) You pick a couple mattresses you like.
2) You lay on them.
3) You see which one is more comfortable for you, and that is your new bed.
Latex foam mattresses are currently gaining popularity over the traditional spring mattresses. A latex mattress provides a precise amount of firmness and pressure for a person.
Tempur Pedic mattresses are made out of memory foam instead of having springs inside of them. Many people find this more comfortable. Tempur Pedic is the most well known, with the best reputation, company that sells memory foam mattresses.
One might find reviews on different type of mattresses on the consumer report website. Also major store that sell mattress such as Serta, Sleep country and Leons will have customer reviews on the official websites.
This site offers a great overview of standard mattress sizing: http://quilting.about.com/od/quiltsettings/a/mattress_sizes.htm. In terms of where to buy pillow top mattresses online, amazon.com is a good resource because many vendors sell mattresses on Amazon, so you can see mattresses from many different vendors all in one place. That way, it is easier to compare prices. Also on amazon.com, you can compare reviews of the product you're looking to buy. Since you can't try out the purchase if you're buying it online, reviews are very helpful.
Some mattresses but not all have fully organic material. Check the tags of the mattresses to be sure.
You can try checking out sleepies for seally mattresses as they sell all designs and makes of mattresses to fit the customers comfort You can also try checking Macys or jcpenny as they also sell mattresses
Visco foam mattresses are often referred to as a "Memory Foam" Mattress. These Visco mattresses are better than other types of mattresses because they adjust to the persons body that lays on the mattress.
Tempur mattresses do cost more money but then again all mattresses cost money, however the tempur mattresses are made of really really good quality to make sure you have a great night sleep. If its worth it then go get it.
Organic crib mattresses are made with only organic materials including organic cotton. They are likely to be kinder to skin and more hypoallergenic than regular crib mattresses.
All places that sell mattresses always claim to have cheap mattresses for sale to improve their sales. Many stores have yearly sales to increase business.
because diffrent companies make diffrent size mattresses or diffrent size bunk beds
Well, there are a lot of different types of mattresses, in general there are - innerspring mattresses (has the longest history and has the largest market share) - memory foam mattress - latex mattress - air and water mattress Each of them has its own pros and cons, you need to decide which one you need based on your own budget and your main concern with sleeping. I have include 2 links where you can look into more details about mattresses and different types. Hopy my answer helps