Crib Mattresses?

Da

I'm looking at Walmart and Target mostly, trying to go find a good budget option. The crib we will most likely buy says that a 27.5" W x 52" D mattress will fit, which I guess is just a standard crib mattress? Not really sure what to look for. I've read that firm mattresses are supposed to be safer, but there are so many different materials, brands, and depths to choose from. Any suggestions??