biodiesel tank?

You are talking about a conical bottom tank. That's pretty much what they're called. It just provides some restriction to volume. During producing biodiesel it is required to make several separations based on phase (not liquid/solid, but different solubilities of liquids). The cone bottom of a tank allows a lower volume of liquid to show at a higher level. By doing that, you can watch more carefully when making this cut to avoid keeping to much junk in your process or throwing away to much good biodiesel.

If you want to save some cash on that and are only making relatively small amounts at a time (100 gal or less) take the normal vessel you are using now and put it on an angle, maybe 30 degrees. As long as you are able to tap and place a new drain at the new low point you have accomplished the same thing.

Note: if you decide to use this method make sure to provide support for the tank, I don't want you spilling oin and diesel all over the place.

You can take any white plastic 55 gal. drum, and drill hole near bottom,to drain the water soapy layer.