ORIGINAL: MinnFlyer
Kitbuilder, OS and Saito sell those flex pipes. Check Tower and Horizon.
Cappaj1, that will work just fine, but wrap a piece of copper wire (or other soft wire) around the muffler and attach it to the engine mount just in case.
If you want something pre-made, you can get [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXB858]one of these[/link] which will fit your 1.25" muffler.