Need for proper air filter for bioreactor?
Well then you need to get a filter specific for air filtering.  I recommend Polytetrafluorethylene PTFE membrane filter.  Now the only issues are size and "how sterile."

There are a lot of vendors out there that supply these.  If your institution has an agreement with VWR, they certainly have them.  If not, I have used Sartorius in the past, but I don't have any particular brand loyalty to them. 

Let me know if you need any more help.