Mattress for Baby Mod Modena/Mattress rec's?

symphonyofblasse@hotmail.com

I'm planning to get the Baby Mod Modena Fixed-Side crib from Amazon, and have run up against the dilemma of a mattress. First because I'm not really sure what type of mattress to get, and second because I'd like first-hand experience with a mattress that fits this specific crib.

Any specific recommendations? I'm mostly concerned with firmness, and would prefer it to be more natural (can't afford organic, but without harmful chemicals would obviously be ideal). 

Thank you!