How does Channel 5 find the people who live on isolated islands for their show?

When you are an expert like Ben Fogle might be, who likes to travel and has interest in meeting such cut-off tribes, then it is actually not that hard.

Being an isolated tribe does not mean that they have never had outside contact its just that they don't want to be integrated in the society as large.

There might have been travelers explorers who first came across such tribes. Even planes/helicopters flying over dense jungle could spot them as well. Then I am sure people write about these things. Also because of the interest generated from the general public (resulting in shows like Ben Fogle: New Lives In The Wild) or just out of personal interest the explorers would have put in an extra effort to get to know them. So that is where we get the information.

As far as finding such tribes go, try googling it and you will come up with several results. Here is one case example of how the process might look like:

1) You Google un-contacted tribes and find there is one in Andaman Islands.

2) You find out if you know any locals from this region who could tell you more about this area.

3) You pack your bags and fly to this region, consult your local guide and ensure that these tribes are safe to visit or try to find someone who can initiate contact (read: you might need some special language skills for contact).

4) Once contact is initiated you try to negotiate with them to let them film you by offering them something in exchange to what they might need (like some special food or something or even money).