If you are running a smaller hps bulb those candle chimneys at the big craft stores fit the bill and are a lot cheaper and instantly obtainable locally.
A big metal halide won't fit them though, but thin hps bulbs no problem.
People find those candle chimneys for as low as $5, up to $10 or so is all they cost.
Caveat: If you run over around 250w they will break from the heat of the bulb ****if the fan is not running all the time****, they are not like the pyrex "bake-a-round" that can stand any crazy heat you throw at it. That's fine if you are careful though, even if your fan went out and you lost one, they're so cheap...