Amazon have them, they're made by diamond toys select and they're around £40 ish each but can be more depending on availability. You can get the original ship from the series, the refit from the first 2 movies, the a,b,d and e. The hardest ones to get are the one from the series and the refit from the first 2 movies.
The finish on some of them can be a bit suspect, i had to go through 2 enterprise b's til i got one that was acceptable. The first one some paint was missing from the ship and it had some odd smudges. Just search for diamond select enterprise and you'll find them, roughly 12-16 inches in length depending on the ship.