I think you get it by the same guy after he's done showing you around in cherrygrove city. It's just the same thing in heartgold.
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it is on your pokegear when guide gent gives it to u
I advise you to use your map.
first you have to find it on the map then you just follow the map until you get there its as simple as that
there is no map. just use the pokegear. to get lugia you need to get a certain item first.
The Town Map Card can be received after getting a tour around Cherrygrove Town by Guide Gent. He is a old man who is a the front of Cherrygrove Town when you first go there.