Depends where you are. Heat-pipe-based heat sink assemblies are usually
custom-made, and you need to post your geographical location if you
want somebody to tell you about a supplier who is close enough to you
to be useful.
The one name I can vouch for (we acknowldeged them in a published
paper) is Isoterix of Woolmer, Northumberland, U.K.
The American Insititute of Physics buyer's guide lists two sources for
heat pipes, one of which seems to be a mistake. The other
http://www.norenproducts.com/
seems to be pretty much exactly what you want., if Menlo Park in
California isn't too far away. Maybe John Larkin has used them ...