I wasn't able to find aspen, so i decided to give a try to corn cob and it seems to works very well!!! Mices sometimes try to chew it but they give up fast, i will check often to make sure they digest properly and maybe try to check in farm and horse store to find aspen like suggested Taceas.
For now here are some pic of my setup with corn cob bedding, it's less comfortable but they have a lot of nesting material to make hides and i think health is more important than comfort...