Do you need the whole clamp or just the little clip that holds the tube to the clamp?
The little clip piece that goes around the tube is an off the shelf cable restraint, most better electronics stores will have them, it is the 1/4" size, though white (natural, kind of a translucent, off white) is generally more common than black.
What I have done in the past to avoid using the clamp is just dremel out a slot in my overflow box a little so I could squeeze the hose through and it held very tight. I personally try to avoid all the clamps and stuff if I can, just be sure it is secure and the hose is not touching the water. You just need to widen a stock slot in the overflow a little to do this.