I think the problem with bark from your yard is that you might bring bugs home. (Bugs!). The second would be that the bark you have may be of inappropriate size for your particular orchid.
You could probably sterilize your bark and run it through the chipper a few times until it got to the right size. Also you would need to add some perlite and maybe charcoal to your bark. It may be easier to buy a bag. But buy the right bag for your plants. So, as Jim stated, we need to know what kind of orchids you have.
Daisy
