First of all, you can do a google search for the company by name.
You can also check the Yahoo or Google directory, or the white/yellow
But the most effictive solution will probably be the business register
of the country/state the company is in.
For the US, the Thomas Register should list all N-American companies,
you can find them at: http://www.thomasregister.com
You can also use the business center of the Superpages (yellow pages
If the businesses are in the UK, you can find them at:
Of course you can organise a mailing to all adresses you have, but
that would be quite expensive (depends on how much businesses) If all
else fails, I should pick up the Yellow Pages and search for the
company the old-fashioned way.
I hope this helps you,
should you have any more questions, or need anything else, feel free
to ask !