I presume you mean for compression testing an engine? There are probably many places on-line but if you are in a hurry then your local motor factors should have one (even a halfords) or somewhere like machine mart. Check it will have the right size fittings for the spark plug holes on the engine.
Gas engine ones are easy to find at auto parts stores.
Diesels are considerably higher pressure and the gauge may or may not be in the appropriate range.
