For all I know he does the research (book research, not data research) himself. From as far back as his Senate days he has been in contact with some of the top scientists and has gone all over the world to see what scientists are finding. He worked to declasify the evidence on the thinning of the polar ice sheet (US Navy kept decades long track of that for their submarines) once he learned about it...and my wife remembers when it was declassified and how shocked many in her field were at the rapid thinning (40% thinning in a rapid period). So he has all the resources to talk to the scientists directly. And some of the scientists he shows in the movie are well known to my wife (e.g. the world's expert on glaciers, etc.) In fact, much of the data he uses is available to almost anyone and often speaks for itself even to non-specialists.

However, I also assume he has advisors. I don't have the book handy, but the companion website to An Inconvenient Truth doesn't list advisors. I am sure the credits of the movie do.