A source inside the Richard J. Donovan Correctional Facility tells TMZ ... prisoners are already lining up and calling dibs on the Menendez brothers' property on the inside ... all in the hope of scoring a famed memento or two.

We're told everything -- from clothing items to their TVs -- is up for grabs right now, despite that being a prison no-no.

Our source says that technically the siblings aren't supposed to give anything away ... but that's not stopping inmates from forming a line under the radar in the San Diego facility.

Despite all the renewed attention on Erik and Lyle's case, we're told the brothers have been unusually quiet and reserved this week. It's obvious that they've been "nervous" about what's to come from the D.A.'s office, our source claims ... since their fates lie in Gascón's hands.

That said ... the infamous duo have been promising fellow inmate pals that they'll do what they can on the outside to help their cases!

Gascón held a press conference earlier in October, where he confirmed his office is reviewing new evidence in the case ... which could result in a resentencing or a new trial, potentially setting them free after spending almost 35 years behind bars.

Erik and Lyle are currently serving life sentences without parole for killing their parents José and Kitty Menendez in 1989. The infamous brothers were tried twice for the murders ... their first trial resulted in a mistrial over the jury's deadlocked decision.

Erik and Lyle's accusations of sexual abuse against their late father were not allowed to be presented in their 2nd murder trial ... in which they were ultimately convicted.