We happen to use cypress Mulch (home depot, or a local garden shop)
Anyhow we found it to be easiest to use and fairly inexpensive (we need to get substrate for 50 or so cages, so that seems to work great for us.
PS animals in isolation / quarenteen are always kept on either white paper towel or unprinted newspaper (easiest to get a fecal that way)
Ok just my two cents
RUSTY