Hi Bella,
I got both a wooden cot and angelcare sound and movement monitor and I bought my baby's mattress from babycity. It's called Healthtrex, in 2010 I paid abt 600 and it gots holes, to allow for circulation as and it has a mattress cover. I also use a wedge to elevate baby's head from the same "company" and it works great with the monitor.
The wedge also has holes, so it complimenets well. In case u buy a new mattress without holes elsewhere, my antenatal nurse told us that u could take it to any bed shop and they'd punch holes in the mattress for you