I believe that's the way it used to work. I can't seem to find a safe-looking site to download the map from. It would also appear that the official forums were taken down some time ago. Its been years and years since I last played online (as in around 10 years ago), and I believe that if you played a custom map online, the game would save that map in your game for you to either play offline or host yourself. But I'm not sure that's 100% accurate, and besides blizzard appears to have started updating the game again, so its possible they may have changed that.
The reason I want to know this is so I can look up the items in dota 1. I can only find information online for dota 2, which isn't identical to dota 1. If I could get a map through the game that I can play offline, I could just open up the game offline and look at everything that way. As for playing that match, I was planning on playing a support so I could get away with not having to buy too many items. I don't want to sabotage everyone else's game to do this.