I suspect that Joann's would be a good place to
start.  Ohterwise, you can make your own (Bob would...):

http://blog.modernmechanix.com/category/how-to/page/8/

Bob Hoover, rest his soul, wrote of this years ago. 
If you have The Sermons, look up "The Silk Thread Trick."

Here's Bob's comment on a homebuilt forum: 
http://www.aviationkb.com/Uwe/Forum.aspx/homebuilt/3772/Engine-case-sealer
About 3/4s of the way down.

Short version: 

#3 Permatex ("Aviation")
Just color the mating surface with it.
Let sit for 5 minutes
place 00 silk thread in the Permatex 
Double strand. One inside the bolt holes, one outside.
Do not cross the strands.
Bolt case together.
Trim any extruded material.
Let sit 24 hours before adding fluids.

The thread crushes and becomes a gasket. (That's why
SILK. Synthetics don't crush.)

Jim