This is not actually new, but it's now more accessible. In the Source Review page you'll notice additional buttons "Domestic: ALL, US, UK, EU, CA, AU" that can be used to sort sources by their base of operation.

Keep in mind that the output isn't complete, it only sorts websites that have been tagged as such (domestic or not), by their official representatives.

More sorting is available if you click the INFO tab in the source's review page.