Instructions
Ask friends, colleagues and neighbors for foreign stamps. Even in the 21st century, plenty of people still receive letters and postcards from abroad. Check with local charity outlets. They often collect foreign stamps and parcel them up by country or region. Pick up cheap starter albums from yard sales--these will usually contain a cross-section of countries.
Go to an online auction site, most of which which have large stamp-collecting categories. Browse through the subsections until you find the country you're interested in. This is a good place both for the novice to snap up job lots and for the more experienced philatelist to fill gaps in his collection.
Do an online search, using the words "philatelic bureau" and the name of a particular country. Virtually every postal service in the world now operates a site selling directly to the collector. These bureaus are great resources of news about current issues and information about their back catalog.
Visit a specialist philately store. The dealer may have just what you're looking for and be only too glad of an opportunity to part with it.