Aspen, cypress muclch, bed-a-beast etc. can all be found at most pet stores. You might want to check your mite problem. Although is possible that they are comming in with the substrate, they may be hanging around the snake enclosure and being discovered when new substrate is put in. Remember that mites will leave the animal to lay eggs, which can remain dormant for some time.
You could always microwave the bark and/or put it in the oven for a while but carpet or paper towels/news paper is a better choice anyway (and easier to replace)