Sealy mattresses (or other recommendations)

Our boy transitioned to a cot a few weeks ago, we bought the Childcare cot from TArget and bought the mattress from there. We didn't put it together until he was getting ready to move out of the bassinet so I hadn't thought anything of the matttress. When we put it together I felt it might not have been firm enough. But it seemed okay when we first put him in it, but I've noticed that it doesn't seem firm, and the other morning in the low light whilst feeding him I noticed that his head had left an impression in the mattress, it hadn't sprung back, so I'm very sure that it is not firm enough and as soon as possible I'm going shopping for a new mattress. I was a bit freaked out when I saw that. Thankfully he cant roll over yet, I fear if he could, that his face would sink in too much and air flow wouldnt be good.

So I noticed that Toys R Us are having a sale and the Sealy mattresses are on special. There are 3 types that they sell, not sure which is best. Has anyone used a Sealy? Do they have a good firmness? I imagine given the name they should be suitable and spring back quite well?

Thank you.