Ryan, Reese's, and Ed's John Deere Snowmobiles - Also lots of great info including a Fire series manual
Andy's John Deere Snowmobiles
Tyler's John Deere Sleds
alt.snowmobiles.old - Old sleds newsgroup.
My posts to rec.sport.snowmobiles
Vintage Sleds.com - Very cool site. Check out the sled in the "Rare Birds" section with a 351 Windsor!
Central Snowmobile - Huge selection, free catalog.
Image Enterprises - Lots of parts, but pricey