I just bought a Hampton Bay ceiling fan but I need need a 24" extension for it. I got one from Home depot but the diameter was to big. Where can I get one to fit model #157 734?

Most Hampton Bay fans use a 3/4" downrod (1" OD). It's possible your fan uses a 1/2" downrod, what model do you have? It's also possible you're just not installing it right.

What diameter is the downrod that came with the fan? The measure that's used is the diameter of the hole in the center.