Tupac Shakur's expansive posthumous catalog could grow even bigger ... if any hardcore collectors have the cash to unearth some of his pre-Death Row Records tracks!!!

Momentsintime.com is selling the lyrics and audio capsule of unreleased Tupac music stemming from late 1990 to early 1991, when the late rap martyr was recruited for his budding talent.

Tupac composed and recorded an unreleased album for a group called Jesse and the Kidz ... which never dropped due to the unspecified death of one of the band members.

Mr. Makaveli himself commands three of the ghost album's songs as the lead vocalist, and the lyrics have yet to set to see daylight ... though you can take a peek at some sample sheets.

The Tupac treasure will run you a cool $250,000 and is available only for private sale.

That means you'll be able to listen to your heart's content, but will have to congregate with Tupac's estate -- which controls distribution -- if you want to release it publicly.