Are you 100% sure? I thought we didn’t have one and it turned out to be in the kitchen pantry behind a plastic shield. I had looked for it with a plumber when we originally bought the house. Years later another plumber told us we had to have one because of code (in my state in the USA) and since the house was fairly new (less than 20 years old).
Mine was built in the 80s. I spent the morning crawling around my crawl space. There may be one down there but I can’t find it or it’s beind the sewer line I can’t get past, which just means I don’t have one. There is a knob in my garage which I have no idea what it goes it. I have turned it till it won’t turn both ways any nothing has happened that I could find.
There is a chance it’s “outside” in some sort of deep box but that seems unlikely unless you are in the south.
You need a plumber unfortunately and if you really don’t have one it might be a grand or more. Schedule with water company to turn it off at the street, cut, add a good modern shutoff in a useful location (which might require changing layout) and so on.
Good luck, but I would not do it myself based on the comments so far. Just fyi.
Don't use a ball valve unless you can trust your self to always do it slowly the shock to the system if you slam it closed can break old lines. The proper valve to install is a globe valve.
There is a knob in my garage which I have no idea what it goes it. I have turned it till it won’t turn both ways any nothing has happened that I could find.
Ohhh, THAT’S why my lights kept getting brighter and dimmer!