If you are referring to the lay box I use coconut fiber and it works fine for me. If you mean substrate for their enclosure...I use shelf liner and paper towel.
Yea use paper towls as substrate... In the lay box you can use Moss,Vermiculite,Coconut fiber,etc etc... I use a coco fiber/vermiculite mix I find it works great because i go away a lot so my eggs do dry up from the coconut fiber becoming dry.