We're told not too long ago Spencer was at the Malibu house he once shared with Heidi ... moving out some of his stuff.  Sources tell us Spencer came upon a camera with XXX video of Heidi and Karissa -- and the light bulb went off.

Sources say it was then that Spencer decided he could make a fortune selling his "library" to Vivid Entertainment -- much of it featuring naked, fornicating Speidi.

We could not reach Heidi and her reps were mum.

As for Karissa Shannon -- she tells us such a tape does indeed exist, but she's not convinced Spencer really has it. Karissa says if the tape ever sees the light of day, she'll sue the pants off him.

Then again, he might enjoy that.