Bio balls are cheap. Most C02 reactors have some kind of media in them,
amd some do come with what amounts to bio balls in them. The Aqua Medic
reactor 1000 for example. There are several plans for DIY reactors on
the internet, people in my forum have come up with some that may give
you inspiration or be of help.
http://aquabotanicwetthumb.infopop.cc/eve

Robert H
www.aquabotanic.com