There are some free Barbie patterns online. I found these:
http://www.sewingandpattern.com/proj...ies-nightgown/
http://www.sewingandpattern.com/proj...bies-peignoir/
You could probably make a nightgown by modifying this tutorial:
http://www.craftinessisnotoptional.c...-tutorial.html
And here are some tips on designing your own:
http://web.archive.org/web/201004250...g.php#Patterns
One thing about Barbie dolls -- they have changed over the years. A vintage pattern may not fit one of the newer Barbie dolls. In fact, the latest incarnation has Barbie coming in a variety of different heights and body styles. Your best bet may be to buy the exact Barbie doll that is in need of clothes so that you are sure everything fits.
Edit: Here's a link to a photo of the new Barbie doll bodies:
http://fashionista.com/2016/01/barbie-tall-curvy-petite
(The tall, petite and curvy are the new body types. The 4th is the "original".)