As you know, "Wire", used as a verb, meant "to telegraph"; for you youngsters out there, that was the 19th century's version of the internet. "Paladin San Francisco" was his 1870s-style email address.
His nom de guerre, "Paladin", is a medieval term which means something like "Virtuous Knight".

This is how he got his customers, after reading about an interesting case in the papers, he would mail a copy of his card to a potential client, who would then send him a telegram requesting his services. This was never actually explained in the show, it was just assumed that the viewers would know that's how he got his business.

I have a copy of Paladin's business card as wallpaper on my machine at work, which I have to continuously explain to my younger colleagues. It's odd, but the phrase "Have _____, Will Travel" has now become part of the language, even though most people today don't know its origin. It has become part of many movie titles, etc. Even Heinlein's book "Have Spacesuit, Will Travel", owes a debt to this show.