By inducing warm air into a crawlspace, heated or worst, unheated, will cause condensation problems. The next thing you know, you'll be writing to us as to how you can fix your mold/mildew/rotting joist problems!
If you run your duct through a warm 'space', then right outside, you needn't insulate it. HOWEVER, if you run your duct through an unheated space, you need to insulate it. Otherwise, you'll have that moisture problem I mentioned earlier, in addition to water in the duct possibly running back into the dryer.
You can try your local Plumbing Supply House (in the Yellow Pages under the same heading), or your local Home Center. My best to ya and hope this helps.