* Stake sale could be mix of public, private investors -sources. : FRANKFURT, Sept 26 (Reuters) - German public-sector bank NordLB, which aims to boost its capital by about 3 billion euros ($3.5 billion) to cope with bad ship loans, is considering a stake sale to a mix of private equity investors and savings banks, sources close to the matter said. NordLB said in a statement on Wednesday that it was working on a restructuring plan that could include bringing in new investors, adding it mandated JP Morgan and Houlihan Lokey to help it negotiate with potential bidders.