So I would required to stock the following power supplies:
UK: 240V 5V
EU: 240V 5V
USA: 110V 5V
Rest of World: No idea of the mains electricity in every country.
You don't you just need a standard switch mode 5V regulated supply with figure 8 mains input (cant remember the official connector name). That will deal with all input voltages (except maybe North Korea) local user just supplies standard figure 8 lead for their locale.