Buying a mattress

Earlier I had posted about crib vs bed...My Dh and I have agreed to just get a mattress for the baby's room..since we will try to co-sleep..we are not wanting to waste money or other peoples money on cribs etc. Now my question is with all the toxic mattresses and making sure they are pvc free etc. We are not sure where to get one that is not an arm and a leg...our Cal king was $500.00 and DH is not going to spend that on a double bed...but all the natural..cotton..Latex are so expensive...what did others do about this??? I know I can wrap the bed etc. What do I do about a mattress that is new if we can not afford a natural one??? Do I air it out outside??? I live in Ga...between Athens and ATL...so any help would be great...my MIL wants to help us buy baby furniture and I want to get this situated before we go out to get anything.....she does not know we are NIXing' the crib idea...uh oh So should I go with a double or a twin...it will be for the baby's room...I am not putting a mattress next to our bed (our bed is really high..and we do not have storage for it to break it down) Thanks...and sorry so long..peace mama's and papa's