If you have the name of a specific document, try a Google search. Otherwise, you could go to the official websites of overseas governments or parliaments.
Many websites offer a search facility but it is often more effective to search a website via Google. Use Google's Advanced Search and limit by site or domain (e.g. whitehouse.gov). You could also select PDF as 'File type', to retrieve whole documents as opposed to web pages.
Few libraries in the UK have extensive holdings of overseas official publications and libraries that do may not include items on their catalogues.
The two libraries have good collections are The British Library and the London School of Economics. We recommend that you email a named official publications librarian for further information on the collections and to ask about access arrangements.
Do not visit another library without checking that they have what you need and that you can get access. To visit the London School of Economics, you should join SCONUL Access. To use the British Library you must register for a Reader Pass.