This video is just over a year old. There is some good reporting being done, I'm just not in touch. Searching itv dot com website turns up lots of results for Exposure (the name of the show), but nothing for the name of the video. Likewise, National Geographic only has the last segment. Thank god for YouTube.
The prime ingredient in the bombs is ammonium nitrate fertilizer. Some people say it is coming from Pakistan, Pakistan says there are other countries around Afghanistan that could be the source. Doesn't really matter. It would only take 1% of Pakistan's production to supply the bomb makers with all they need. From an ABC news story from January of this year:
"It's like narcotics crossing the U.S. border," said the senior U.S. military intelligence official. "We don't know if we're getting 1 percent or 10 percent that's crossing."