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Ronnie Ortiz-Magro's baby mama, Jen Harley, is in the market for more junk in her trunk ... and she's gonna do it with the help of some fat donations.

Harley's set to get fat injections early next month, however, it won't be her own blubber. We're told doctors take fat cells out of other patients, clean them up, and then inject them into folks looking for a thicker caboose.

L.A. Dr. Jaime Schwartz will do the honors ... he specializes in performing the procedure on women who don't have enough fat in their bodies to move around.

We're told the process takes about an hour and a half and is pretty much pain-free. Jen will be awake during the whole time.

BTW the procedure costs around $25k.

When all is said and done, Jen's ass will be fuller, rounder and better sculpted.

The timing makes sense for someone looking to move on ... Jen and Ronnie just had a nasty breakup after a domestic incident in L.A. Ronnie's been charged with 5 misdemeanors for the incident, including domestic violence and child endangerment.