Mine are cellulose. If you want Aspen shavings you may need to select another cooler unless you can find custom pads.Did some googling, and aspen shavings are still king despite low lifetime.
MasterCool Media/Pad Set MCP44-PAD - The Home Depot
Supplement an essential addition to your room with the choice of this MasterCool Media Pad Set.
www.homedepot.com