It Depends . . .
on what you're looking for.
I've got a North End hobby shop that has the plastic tubing I like, but it comes in a "kit" for radio controlled airplane cabling. It's called "goldenrod", and it's a slightly flexible golden yellow plastic tube that resists breakage and is the smallest OD I could find.
That being said, I tie most of my tubes these days on very narrow OD brass tubes also available at that same hobby shop. About the same OD as the plastic, I have to cut each tube to length and smooth/polish using a Dremel tool and assorted attachments.
Sounds more involved than it really is - I can probably finish 2-4 tubes in less than 1 minute.
The advantage to brass is that, for everything except pure floaters/skaters, I want the fly BELOW the surface of the water. Unfortunately, when tied on plastic tubes, most flies tend to skate in all but the gentlest of currents.
This doesn't really matter if you're fishing a sinktip, and I still tie all my Intruder-like flies on plastic tubes. These narrow brass tubes seem to give the fly about the same density as a moderate wire conventional hook.