Check the phone books. you'll see the numbers for main HQ's of companies here. If not, search the Internet. Many companies have websites.
Also, the goal is still to meet the boss. however, it also pays to be on the secretary's good side. the secretary is not the goal, but in order to get to the goal, you have to pass through the secretary. in the course of trying to meet the boss, you will always invariably run into the secretary. Now, some offices, you can immediately talk to the boss - but if the boss does not know you, or does not consider you important enough (sad as it may seem, it happens), then you will be relegated to the assistant or the secretary...so, be nice to them. they might give you the chance.
Also, do not rely too much on friends of friends or "pare, kilala ko si ________". You never know what the real score is between your friend and the contact. better to introduce yourself independently first, and then use a connection. using the connection first is often a vary bad idea.