Sterns and Foster Mattress set

I purchased A very Expensive King size Pillow top mattress set from Art Van Furniture In Ann Arbor Mi and have had the set less 1 year. I complained about the Mattress Sagging 5 months ago and they said it was only sagging One Inch and they could not do anything unless the Sag was an Inch and a half, I had them back Thursday 12-8-05 and the sag had increased one fourth of an inch still no help. Meanwhile my Poor Back is Suffering, Is there anyway to contact Sterns and Foster???