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
Keep your evaporative cooler operating at optimal levels with our OEM replacement media set, MCP44-PAD. The high-efficiency rigid cellulose media provides excellent saturation to produce lower temperatures.
www.homedepot.com