You could decrease evenly across every so many rows, or dec 2 sts every other row. You may have to experiment on a swatch to see which gives you the dec rate you like.
from Last Minute Knitted Gifts has bell sleeves. You can get the book through the library. If you go with that on, many people added more rows and decreases at the neck; it comes out fairly wide and low cut.
Patons has a book with these sleeves, Patons #837, Next Steps Two and a free one
And a couple below from Crystal Palace - you can adapt the sleeve shaping portion and use something else for the body if you don't like the overall sweater.
There's a few more in magazines that came up in a Ravelry search
I did, you can look through them.