As far as where to purchase a mattress or anything for that matter discuss your intended purchase with your salesperson and make a judgement call on who is well informed and who you feel comfortable with. Find out about how the product is made and what is inside it. As far as mattress warranties. In the bedding industry if you are purchasing a known quantity like a Tempurpedic for example the warranty is set in stone. In other cases retailers can choose labels and in some cases the length of the warranty. I don't think you could say for certain for example that a mattress with a 15 year warranty is always better than a mattress with a 10 year warranty. Also have a look at some of the smaller manufacturers. They may have a mattress with better components than the known brands for less money.

I bitterly remember a mattress, high end can't remember the brand, with a 15 year warranty.... And we had a "certificate"! After 7 or 8 years a spring broke and poked through, so we, knowing it was guaranteed for 15 years and we had a "certificate" went to the supplier and asked about them making good. They explained that the "guarantee" only covered a percentage of the use.....since we had used the mattress for which we paid about $350 for half of the guaranteed life, they would only allow half or out purchase price on a new mattress of the same brand.....which cost a lot more!. And If you have a mattress that has broken springs after 7 years.....are you really going to buy another of that brand?
The mattress industry is full of fraud. I second Freethinker....small manufacturer.