Try the Fjord1 ferry:
It's part of the Nutshell route.
Edit Addenda: above link was for a specific route & date which will not display post travel date. Use english page for Fjord1:
Then enter Flåm and Gudvangen in the from/to and desired date for travel.
Flåm - Gudvangen Dizzying Mountaintops and Deep Fjords.
The boat tour between Flåm and Gudvangen winds around precipitous
mountains, mighty waterfalls and small homesteads that cling to the
mountain sides. Along the way you can see Aurland, the centre of the
municipality with the stone church from the 1200's. You pass by the
village of Undredal, which hosts one of the world's smallest churches.
Way up on the mountain side lies the Stigen homestead, whose name
comes from the fact that people had to use ladders (Nor. stige) to get
there. Soon you will also reach the Nærøyfjord, a minor fjord
branching off from the Sognefjord, which is the world's narrowest
fjord at only 250 metres at its narrowest point. The Nærøyfjord was
also added to UNESCO's World Heritage List in 2005. From here you can
view the majestic Sognefjord, the longest and deepest fjord in the
world. The fjord is framed by mountaintops of up to 1800 metres. In
the early summer the waterfalls roar over the mountain sides and
breaks in a white foam when the water hits the fjord. In Gudvangen you
may also get off the boat and look around before returning to the boat
on your way to Voss, Bergen or Oslo.