The desirable range is
about 0.008 - 0.012".

It's likely possible to use metal capillary tubing for this, which is
easier to find in small qty, but plastic would be much easier to work
with.

Anyone have a source?

http://www.stiflow.com/tubing_PEEK.htm

Call them up and very politely request a 'sample' of what you think
you need after some friendly smalltalk, they'll likely do it... If
not there are many other sources to ask the same of.

-Paul