First off all, you need to learn the lingo properly.

They're not 'Mercenaries', they're 'Contractors'. Watch the BBC news etc. They'll say "A Contractor working for BP was killed in Iraq today." That's the Mercenaries. The blokes kidnapped off Oil Rigs in Africa? Same thing.

You're best bet is the multinational oil companies, always looking for people. Either that or dig out Sandline or Executive Outcomes from the phone book.