It doesn't sell out most of the time unless you're right at some giant holiday like New Year or Golden Week.
If so, what are my options?
Of course other alternatives include (not in any particular order)
-Keisei Skyliner to Ueno then taxi (or Ginza subway line from Ueno to Kyobashi Station, and walk from there)
-Keisei Limited express then taxi (or same subway connection as above)
-Airport bus to Tokyo Station or hotel near your hotel then taxi
-Narita Express standing room or green car to Tokyo Station then taxi.
-JR Rapid train departing NRT at the top of the hour to Tokyo, this train is a regular commuter train like the Keisei Limited Express.
If you're looking for the cheapest option, and don't mind riding in a regular commuter train or have too much luggage to lug around, the Keisei Limited Express to Keisei Ueno and Subway connection at Ueno is the cheapest.
There are a bunch of other options i'm sure thats out there and I can't think of, but I don't think the Nex will sell out to begin with.