O.J. Simpson's GF -- I'm Shopping a Sex Tape

OJ Simpson's ex-GF was stabbed repeatedly .... with a penis ... and now she's shopping the footage around to porn companies ... TMZ has learned.

The woman at the center of the action is Christie Prody ... who dated OJ for 13 years before he went to prison.

Prody has contacted multiple porn companies, including Vivid Entertainment, in the hopes of selling a homemade sex tape ... but before you get too fired up ... no, her co-star is not The Juice.

We're told the male featured in the video was Prody's rebound guy following her split with OJ.