DT needs to post a $464 million bond to satisfy the judgment against him -- he needs to do it soon ... by the end of the month, in fact. However, his attorneys say he's struggling to come up with the cash.

In a legal docs, obtained by TMZ, Trump's lawyers signaled Monday that they're hitting a roadblock in their efforts to find an insurance company willing to secure the substantial financial penalty ... describing the task as a "practical impossibility."

Trump's team explains they've unsuccessfully approached 30 underwriters to back the bond ... writing, "The amount of the judgment, with interest, exceeds $464 million, and very few bonding companies will consider a bond of anything approaching that magnitude."

The filing says their efforts include "countless hours negotiating with one of the largest insurance companies in the world" ... which they insist shows how obtaining an appeal bond in the full amount isn't feasible.

Trump's lawyers add that potential underwriters typically look for the bond to be backed by cash or cash equivalents ... which makes it practically impossible to use real estate assets to back the bond. His team says Trump would need cash reserves nearing $1 billion for an insurance company to come aboard -- and it sounds like they're saying DT ain't got it.

As a result, Trump's lawyers have petitioned the appeals court to stay the imposition of the massive monetary judgment without requiring the posting of the bond during the appeal ... which would essentially block New York Attorney General Letitia James from trying to enforce collection while the appeal is underway. A decision on his filing has yet to be made.