I have first hand knowledge of where the National Enquirer is on this issue because I am in communication with one of the reporters. They do pay their sources.
Here's the bottom line on this: I have stated elsewhere that I believe that one of the reasons this story has been so difficult to "crack" is that only a very few people actually know the truth, a small number of immediate family and a couple medical professionals and they ARE keeping their mouths shut. I believe that Bristol was kept almost completely sequestered during this period. I have no idea how.
Yes, it's true that the Enquirer has not been able to get anyone to stand up and state categorically that they saw Bristol Palin pregnant during the months of, say, Jan-April 2008. However, it's also true that they have not been able to get anyone to stand up and say that she was not pregnant. Not one person. Not one friend has been willing to go on record with the Enquirer and state: "I hung around with her all the time." "Here's a picture of us at Susie's birthday party which was in March." (Nowadays kids have MySpace pages and Facebook pages. They all have cell phones and digital cameras and are snapping pictures constantly.) From other reports, Bristol Palin was a popular girl (maybe too popular) with friends, and had an active social life. So where WAS she during the early months of 2008?
The Enquirer HAS tried. Don't lose sight of the fact that disproving this would be almost as valuable to them as proving it. They just want news, they don't care who Trig's mom is. Running an issue with a screaming headline, "Sarah is REALLY Trig's Mom" or something of the sort would sell a gazillion issues just as surely as "Bristol is Trig's Mom." A high school student who had a SINGLE datable picture from the period of 3-1-2008 to 4-17-2008 - datable because it's at a basketball game or birthday party - something that can be proved - in which Bristol Palin is NOT pregnant could make several thousand dollars or more from the Enquirer. The fact that they have not touched this issue is proof that they can't get a bead on it, either way.
And that says something.