I am using a heirloom bassinet (I was calling it a cradle, but its size makes it a bassinet) and I too need a mattress. I want to make my own, but haven't found a way to do it, besides using high-density foam. Not sure how much that foam will off gas but I felt better when I read that you can make layers over the foam that will slow/stop any off-gassing. So I plan to air out the foam for several days and then layer it with organic/natural cotton batting and then make a wool soaker pad
layer and a slipcover
for the mattress.
OR, you can buy one from:Colgate mattresses
with different sizesCrib parts and replacement pads
I've noticed that the pads for cradles or bassinets are really thin (traditionally they are), so if I do buy a mattress, I plan to make a slipcover for the mattress (pattern above) with a padded top to it (using quilting batting) and then place a wool soaker pad on top for waterproof protection.
Hope some of this helps you, good luck!