You need to find the main shut off valve in the road, side walk or somewhere on or near your property. They are usually placed in mini manholes with easy access:
But if you're unlucky you will need to get some building plans and try to find where on the plans the supply pipe runs and find the valve at the connection point to the main supply. In most cases it will be near the connection point of the main supply:
but if its not, or if you struggle to close it call your council to help you. You don't want to break that!
NB
It is easy to snap these valves- if its rusted badly apply (known as WD-40, LN-40, HT-40) spray oil based rust release (spray leave an hour try again.. repeat a few times), TAP the valve with a hammer (!TAP!, do not whack it, to help release the valve. Also, usually opening the valve slightly helps to release it then try to close it.
Everybody wants to save money
Builders and utility companies are not in the business of wasting money, which means the main line will be as straight and as short as possible. This is a big help to you, since it will narrow your search. Walk out to the curb shut off, and look back at your house. Try to determine the straightest, shortest path between the curb shutoff and the house. Now you'll have a general idea of where the line enters the home.
Trace the line
In most cases utilities bury a tracer wire in the trench above utility lines, to make the lines are easier to locate later. If you call 811 (assuming you are in the US, though I believe this service is offered in other areas as well), they should be able to send somebody out to locate the line for you. This will show you the approximate path the line takes to the house.
Don't let pipes freeze
Because it's a water line, it will likely enter your house below the frost line. If you can determine the frost depth in your area, you'll have a more targeted area to search.
People are idiots
It's not uncommon for people to do dumb things, especially when they don't know how dumb they are. Water meters and shutoffs are ugly, and nobody wants them in their newly remodeled living space. It's possible that your valve has been walled up, and you'll have to remove some finish material to find it. Look in closets, and keep an eye out for access panels.
If only you had X-ray eyes
There are a few tools that can help you search a wall where you suspect the valve might be. A stud finder should be able to locate pipes in the walls, depending on the stud finder you buy. A metal detector will find anything metal, so that could also come in handy. If you've got a bunch of cash to throw around, a thermal camera may be able to actually see the pipe through the wall.
Where does service enter?
After reading the update to the question, and then reading through building code. It seems that it may be possible that there is not a shutoff valve where the line enters the building.
International Residential Code 2009
P2903.9.1 Service valve. Each dwelling unit shall be provided with an accessible main shutoff valve near the entrance of the water
service. The valve shall be of a full-open type having nominal
restriction to flow, with provision for drainage such as a bleed
orifice or installation of a separate drain valve. Additionally, the
water service shall be valved at the curb or property line in
accordance with local requirements.
According to this, there must be a shutoff valve "near the entrance of the service". However, in most cases the water meter has to be near the "entrance of the service" too. In your case, it may be that the "entrance of the service" is at the property line, and not at the building. So there may actually be no shutoff valve inside the house.
Best Answer
You can replace the handle but you've probably have to take it off a new valve that you'd buy. No reason to do that though, just wire brush the valve and throw a little rust remover on it. That's the main shutoff and there could have been a leak above it ,maybe on another faucet, that just ran down the pipe before or after the insulation was added. So clean it up and keep an eye of it to see if it starts to rust again. I wouldn't worry about operating the valve as they are probably the best valves on the market.