If you didn’t apply for a re-entry permit before leaving the United States you won't be able to get a re-entry permit now.
You will need to apply for an SB-1 visa (also called a “returning resident visa”) at your nearest U.S. consulate or embassy before returning to the US.
NOTE: You can also use a re-entry permit as a travel document if you can’t obtain a passport from your home country. Many countries will let you use a re-entry permit like a passport, and will stamp it with their visas and entry and exit stamps. Make sure to check whether the countries you intend to visit accept U.S. re-entry permits as valid travel documents.
HOW TO APPLY FOR REENTRY PERMIT
To apply for a re-entry permit, you must file Form I-131 (“Application for Travel Document”). This form collects details about your planned trip, your foreign travel since becoming a green card holder, and whether you’ve been properly filing US tax returns.