Van-Cafe has them.

http://www.van-cafe.com/home/van/page_1006_957/test_pipe_cap.html

On Wed, Apr 21, 2010 at 8:02 AM, Greg Potts <greg@pottsfamily.ca> wrote:

> Hi Volks,
>
> I recently installed a replacement J pipe between the Cat and the
> collector on my vanagon after the flange rusted away on its predecessor.
>
> The exhaust ran quiet for a few weeks but recently got noisier. When I
> inspected, I found that the diagnostic sensor port plug had fallen off.
>
> Can anyone identify the thread on this, and a potential source for a
> replacement?
>
> Thanks!
>
>
> Happy Trails,
>
> Greg Potts
> Toronto, Ontario Canada
> 1973/74/79 Westfakia "Bob the Tomato"
> 1987 Wolfsburg Weekender Hardtop
> www.busesofthecorn.com
> www.pottsfamily.ca
>