I have used about everything out there...including carefresh type substrates......But I just recently had a mulch company double mill some pure cypress mulch for me and I love it. Not only does it hold humidity but because it is twice milled, it is soft and small pieces sort of like shreded aspen. the snakes seem to like it too. currently all of my balls are on it and my 1/2 dwarf double het ablino granite burms....oh yeah and my baby green anacondas.
The reason for me going straight to the mulch company is because the cypress mulch taht you can buy at stores 99% of the time the label says cypress blend....after some research I discovered that they are blending it with cedar "as we all know cedar is toxic to reptiles" . So I went right to the mulch company and told them what I wanted "100% double milled cypress", and they were all to happy to give me an entire truck load!!! for only 28 dollars......