Organic Grace carries a cotton and wool mattress and also has a build-your-own latex mattress option. Both are completely chemical-free and cost less than or the same as the regular, popular brands. Their website also has several articles about the chemicals used to make most mattress- scary!
Even if you don't go with organic, at least avoid the tempurpedic-type mattresses (100% latex is fine). I've read from multiple different sources that tempurpedic-type mattresses are the most toxic mattresses made and will off-gas long after the nasty smell wears off (which some say can take months!)
Also, make sure you get a firm mattress. But you probably already knew that.
