walmart has playard mattress too. got mine from there. its was like $30 and they have a ton of sizes so you can get one that will fit right.
if you're worried about comfort, they sell playard mattresses online(amazon, ebay) for about $22-30.
if its to cushy they get worried about lawsuits of babies suffocating. i have had a few pack n plays n all have been some what hard. i would never sleep in one myself.
You can fold a blanket and tuck it under the mattress part to make it a bit softer but extreme plush is def not needed, idk if you notice all baby sleep stuff is firm even the crib mattresses so i would def think its necessary.