It's been 4 years since Katie Couric grilled Sarah Palin about where she gets her news ... and now, we finally know -- the National Enquirer.

The former Vice Presidential candidate was hangin' out with her daughter Bristol in a Kmart in L.A. this weekend ... when she decided to check out the magazine stand.

Palin eventually picked up the latest copy of the Enquirer ... possibly because the cover featured a photo of Bristol under the headline, "Plastic Surgery Shockers."

In related news, Palin also bought some toilet paper.