I started a new game in Yellow. Since I've played it before, I'm not spending as much time exploring and talking to people. However, in my haste to become a Pokemon master, I didn't get the map from Gary's sister! I noticed this near the beginning of Mt. Moon, so I returned home. However, Gary's sister isn't giving away any maps. Are there any other opportunities to obtain a map?
So, good news bad news. Unfortunately, we have to start with the bad news. You likely won't be able to get the map in any legitimate way if she is not giving it to you now. In Gen I, that is the only way you are able to get it. In the remakes of Red/Green (Blue) in Gen III, you are able to get it later by talking to Celio on Knot Island (one of the Sevii Islands). But since you don't have access to those areas in the original Gen I games, you are out of luck... mostly.
The good news: You can cheat. Well, that is to say, if you have the ability to enter a GameShark or Game Genie code, you can still get it. Most emulators offer this ability natively, so if that is how you are playing the game you're in luck. If not, you would need to get one of these devices for your GameBoy. The codes are:
01051DD3 Changes the item in your first item slot to a town map
01057BCF Allows you to buy the town map at a PokeMart