At Walmart, get the Dream On Me Twilight 6" 80 Coil Spring Crib and Toddler Bed Mattress for just $29.99 (was $49.99). It's an 80-coil spring mattress with a depth of 6". At Walmart, shipping is free on orders $50+, or get free in-store delivery.

Compare price to Amazon for $29.99 with free shipping on $49+.