I used stainless steel bellows hose from the high vacuum research world... it's awesome and though it costs many hundreds of buckaroos, on ebay it's cheap.
Try searching " mks bellows " or " vacuum bellows " and you'll find some of the stuff.. but today I didn't see any covered with a stainless steel braid which is necessary to keep it from destroying itself with vibration. I found a piece with SS braid that was 3' long, it worked really well, and it was only twenty bucks!
The flanges that come on this stuff are neat too, mating flanges can be found by searching " conflat flange "
I just buy premade elbows, etc and saw them up, that way I get the flange and some tubing already attached to it.
They use 6 bolts to hold together and a little copper gasket ring to seal themselves. Seems criminal to use such a high tech thing for exhaust, but I love it!!!
All of this stuff mig welds to regular exhaust piping just fine
A warning though- if you're tempted to try the unbraided stuff; it can sometimes can be made of really thin material... and sometimes it is thick and stiff...you never know without touching it. The braided stuff is a better bet if you can find it...
...just another weird option from the boogie....