mattress for pressure ulcer/sore prevention OR pressure-reducing mattress/pressure-prevention mattress OR
anti-pressure sore mattress

Special mattress, used for bedsores or prevention of bedsores.
There are many kinds (water, air, foam, etc.) mattresses, the most affordable and popular are "foam mattresses" -- convoluted (egg crate) foam or eggshell mattress.
Bedsores - pressure sores/necrosis/ulcers or decubitus ulcers
Good selection at:
http://www.rehabmart.com/bedsores.asp

"...To reduce healthcare costs and patient suffering associated with the incurrence and treatment of decubitus ulcers, our revolutionary line of Pressure Reduction mattresses address the prevention of bedsores as a long-term care strategy..."
www.missionminder.com/repcan00-TO/profiles/maderite.html

"... 156 beds or mattresses for pressure ulcer prevention ... 301 studies,
www.ahcpr.gov/clinic/ptsafety/pdf/index.pdf

"... regarding pressure ulcer prevention for the samepatient group ... a pressure-reducing mattress and aneducational ...
publications.uu.se/theses/91-554-4889-5

"... In the development of the Thevo-Soft anti pressure sore mattress, the efforts of ...
www.thomashilfen.de/content/export/homecare_products.htm