Looking for GM parts look-up site & part number interchange site

Besides www.gmpartsdirect, does anyone know of a site that will allow me to enter a vehicle year, make, model, etc., and find GM part numbers for a specific part ?
Other than acdelco.com, does anyone know of a site that provides part number interchange information, i.e., you enter a part number from one manufacturer and get equivalent part numbers from other manufactures.
Thanks John
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