Bunk bed mattress recs?
We've purchased an IKEA NORDDAL bunk bed and I need to order mattresses for it. At the time of purchase we were not able to fit the IKEA foam mattresses in the vehicle along with the bunk boxes, so I resigned myself to ordering something later, and "later" is now. 
Anyone have a bunk bed mattress recommendation? I will need to order online. And I'm fairly certain that IKEA claims I cannot use more than 5.5 inch mattress in the top bunk, although I have no issues going with a 6-inch if that's what is non-IKEA standard, which I believe will limit my choices to foam. Not sure if anything thicker would be still be safe with the top bunk guard rail.
Thanks!
DS- 6/09
DD- 4/12
It's a boy! DS- 2/15