15 gallon tub from Lowe's a few years ago and it looks like it's not available online but you can still find them in the store.
To make your tree collar use a reciprocating saw armed with a fine metal cutting blade to remove the bottom of the tub. Just saw around the inside perimeter. It should remove right along the weld line and not leave any sharp edges.
My little Charlie Brown Christmas tree looks so cute tucked inside my new tree collar. Now if I could just figure out how to get those delicate tree branches to hold more of our ornaments!