'Teen Mom' Farrah Abraham -- SEX TAPE FOR SALE

"Teen Mom" Farrah Abraham got down and dirty while cameras rolled and now a sex tape is being shopped to various porn companies featuring the MTV star ... TMZ has learned.

According to sources who have seen the tape, Farrah and a male partner (possibly her ex-boyfriend) go at it for about 30 minutes ... full-on sex ... in various positions. We're told it was recently shot.

Our sources say the person selling the tape has already shown it to some of the biggest XXX companies and is looking to make a deal.

It's another notch in Farrah's belt ... she was recently snapped boozing it up in nothing but lingerie. And last month she was arrested on suspicion of DUI, when cops say she blew nearly twice the legal limit.

Farrah blamed that on being sick. Wonder what her excuse for this will be?