You can replace with any 18mm petcock (there are cheaper ones on ebay.) The replacement will not be a vacuum petcock so you will need to seal the vacuum port on your bike.
Here's the one I bought and it's worked fine for me. (I think this is it anyway as it was a while ago.)
New Tank Fuel Switch Valve Pump Petcock for Honda Nighthawk 18mm x 1mm | eBay