I'll tell you my secrets but stay away from my supply!!
The company i work for gets rid of a LOT of hydrolic oil. I pick through the cleanest before they dump it.
I also started to give all my friends 5 gallon buckets and whenever they fill them up i go get them and dump them into my drums.
"Shade tree" mechanics are a good source too. They end up with a lot of it with no easy way to get it out. I gave my mechanic a clean drum and every now and then i go empty it for him. He knows to only dump WMO or WATF in it.
