Rick:

I have bought coupler tubing from McMaster-Carr. www.mcmaster-carr.com. On their site, do a search for tubing with PTFE, the search should yield two choices, plastic and tubing, select tubing. Once there, select single tubing. Once there, you can select the ID. The tubing is available in many different ID's. I have selected the 5/8 for my YS headers and they last quite a while. I have also used the PTFE on my OS 160 coupler. The teflon is very rigid and does not flex much in short lengths. The tubing is rated for a max temp of 500 degrees F. As a reference, the part numbers I have used are 5033K9. With regards to the two different size pipes (header and pipe, what we have done is to buy the tuding with an ID that fist the smallest pipe. To get the tubing to fit the larger pipe, simply heat the tubing, it will soften and this will allow you to place the softened and expanded tubing over the larger pipe. Might give this a try.

I have also just purchased some softer tuding, much like the gray or blue coupler from Macs or Aerotrend. I ahve yet to try it but the part number is 52035K38. This only has a temp rating of 275 F. It has been my experience that my couplers (the aerotrend, Macs stuff or this PTFE stuff) last a fairly long time as long as it is shielded from the heat by the header or pipe (i.e. I leave only about 1/16" or 3/32" spacing between the pipe and header. I continue to try the stuff from McMaster-Carr because the have a great selection and it is relatively inexpensive (one foot of the 5033K9 is about $7.50). Might order a few different types and see what works.

Hope this helps.
Ed