The only one I know of, currently.

http://centralhobbies.com/Exhaust/hatheadr.html

As I remember it they don't like vibration too well.
The "flex" part is really to help absorb the vibes and not to "bend to shape".
My own guess and that is all that it is ___(well, almost) is that you'll be fortunate to have it all hang together for more than 3 to 4 flights max.

Frankly, I think that is optimistic!

I think you will be disappointed with this method.
In some circles "crackedheaders" is considered to be one word. []

See if you can't rig one from these parts __http://centralhobbies.com/Exhaust/Exhaust.html
At least the Teflon tubing is more forgiving of vibration.
I know of nothing 'ready made' that will do what you wish___sorry.

Should you find something let me know as my own "engineered to last set ups" didn't do much more than further my frustration.
Some of those required more than casual metal working skills and for all the apparent features, braces, and vibration absorbing materials they simply wouldn't take it!

I have come to the conclusion that you simply have to go to an NMP or Hatori header straight into a Hatori short pipe (muffler). It takes fuselage mods (or design) that some consider extensive.

After you've blown out a few set ups they look pretty reasonable!

These folks have some solutions but I don't think it is what you are after. http://pspec.com/