Is there a place around there that does big truck exhaust?  I always go ask around if they have any "junk" like a pipe that got bent and replaced.  They usually just scrap these pipes, and the part that was closer to the bottom of the cab is usually straight and fairly clean, and you can cut a piece that is plenty long enough to use for a tractor.  And the best part....usually FREE!!! <!-- s:D --><img src="{SMILIES_PATH}/icon_biggrin.gif" alt=":D" title="Very Happy" /><!-- s:D -->