You could use the https://ipinfo.io API for this (it's my service). It's free for up to 1,000 req/day (with or without SSL support). It gives you coordinates, name and more. Here's an example:
curl ipinfo.io
{
  "ip": "172.56.39.47",
  "hostname": "No Hostname",
  "city": "Oakland",
  "region": "California",
  "country": "US",
  "loc": "37.7350,-122.2088",
  "org": "AS21928 T-Mobile USA, Inc.",
  "postal": "94621"
}
Here's an example which constructs a coords object with the API response that matches what you get from getCurrentPosition():
$.getJSON('https://ipinfo.io/geo', function(response) { 
    var loc = response.loc.split(',');
    var coords = {
        latitude: loc[0],
        longitude: loc[1]
    };
});
And here's a detailed example that shows how you can use it as a fallback for getCurrentPosition():
function do_something(coords) {
    // Do something with the coords here
}
navigator.geolocation.getCurrentPosition(function(position) { 
    do_something(position.coords);
    },
    function(failure) {
        $.getJSON('https://ipinfo.io/geo', function(response) { 
        var loc = response.loc.split(',');
        var coords = {
            latitude: loc[0],
            longitude: loc[1]
        };
        do_something(coords);
        });  
    };
});
See http://ipinfo.io/developers/replacing-navigator-geolocation-getcurrentposition for more details.