On Apliu Street in Sham Shui Po, lots of shops sell transformers that will convert between 110V and 220V. (Some models convert either way, others only convert one direction.) The thing you must do is figure out what wattage your appliance needs, and buy a transformer that supports it. A higher wattage transformer is fine, but more expensive. These transformers are small black cubes, a few inches on a side. I think they cost (very approximately) 0.8 HKD per watt of power.
Your other option is to convert an outlet to 220V, once you're back in the US. I wouldn't suggest that for a straightening iron, since you probably want to use it in the bathroom/bedroom, instead of being tied to one special outlet.