Thermally but not electrically conductive tube?

I am looking for a thermally conductive yet electrically insulating material to wind Tesla coil secondaries on. So far the only thing I have come up with is glass. And maybe ceramic of some type. But I am having a hard time finding a supplier, other than going down to Wal-Mart and buying some very large dinner glasses.

Any ideas fellas?

Clay drainage tile
**broken link removed**

If you can find any solid diamond tubes of the approximate diameter you need they'd work well, but I think Mike's suggestion of clay would work well.

Mind you BAKE IT to drive out moisture and seal it with something.

A Google search turned up this company: LSP Industrial Ceramics. Their online brochure says they can supply alumina ceramic tubes in diameters up to 9 1/2 inches OD with a 1/4 inch thick wall.

Jeff