After damaging our own regulator by knocking the gas bottle over while it was attached (since then we recommend using a bump guard), we realized there were probably others like us that needed replacement gauges for their regulators. 
 
Luckily, gauges are easy to replace. Just un-screw the gauge and then screw in the new one. These regulator gauges can also be used on many other devices as the 1/4" NPT thread is fairly standard and it will suit most regulators.
 
These are:
Brass Construction
1/4" NPT to suit most regulators
 
Annoyingly Micromatic use an unusual Left Hand thread so this gauge will not fit their regulators, but almost all other regulators will be compatible - it's worth checking first.