Have always used wax for fluxing but after reading numerous posts, I've decided to go with sawdust. Problem was I don't have any sawdust, LOL. I don't do many wood projects and don't see any in my immediate future (still recovering from major surgery). So I drive to Lowe's Hardware (like a Home Depot) and asked several workers in the store if they had sawdust. They sent me a few places to look but no sawdust. Then I remembered the big saw they use in the store to cut wood for customers. I walk over there and look, all the sawdust is sucked up by an internal vacuum. A worker from that area saw me looking around and asked if I needed help. I told him that I needed to buy some sawdust but Lowe's doesn't sell it. He walked in the backroom and returned with a 5 gallon bucket partially filled with sawdust and told me to help myself. He said that its a pain to cleanup because the commercial saw makes such fine sawdust and the store just disposes of it.
So, if you are like me and don't have sawdust laying around in your shop, drive on out to Lowe's hardware or some other store like them and ask.