Your serial number location will be dependant on what kind of shock you have. 

For DBinline, DBair IL, and DBcoil IL, they will be located on the under side of the valve body below the HSR adjuster:

*please have a decent photo taken of this*

For DBAir, DB Coil, DBair CS, DBcoil CS, DB Kitsuma Air, and DB Kitsuma Coil, your serial number will be located on the back (opposite side of the reservoir) of your valve body  right below the eyelet:

*please have a decent photo taken of older CS*