If you are talking about a TPR valve (Temperature and Pressure Relief) then they are readily available from Plumbing outlet shops – but be sure to purchase one with the correct settings for pressure on it.
If you are talking abou the gas valve then I am NOT going to tell you where and how to get one of these as playing with gas controls must only be done by professionals to ensure safety and correct pressure settings are undertaken.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians