I would begin a search of eBay. Ebay frequently lists "Duramax conversion packages" for sale that contain all or most of what you'd need. If you know someone in the auto insurance business, ask them to help you locate an insurance salvage auction in your area, looking for a wrecked Duramax/Allison equipped truck. Local retail auto salvage yard prices usually discourage conversion prospects, but do remain a potential source for at least some of the pieces.

Failing that, the following auto salvage centers have helped with conversion packages in the past:

Schram Auto Parts
Waterford, MI
1-800-462-9292
Mason, MI
1-800-292-1032
www.SchramAuto.com

Stricker Auto Parts
Batavia, OH
1-800-582-5405
www.StrickerBros.com

G&R Auto Parts
Oklahoma City, OK
1-800-678-3377
www.GandRAutoParts.com

Nordstrom's Automotive, Inc.
Garretson, SD
1-800-272-0083
www.NordstromsAuto.com

Spalding Auto Parts
Spokane, WA
1-800-366-2070
www.SpauldingAutoParts.com