<<I got the Al-ko shocks that the bus depot carries.
 Trouble is, when changing them, I need to replace the washer and
damping ring (bushing?)
 I don't have these things at all).
My shocks from the bus depot didn't come with these.
Where can I get these? Does somebody have a part number?>>

Take a trip to your local FLAPS.
They'll have the bushings and washer's

High end shocks come with new bushings and washer's.
( Gabriel are high end? The bushings and washer's are included with them
)

You just have to inquire about this nit picking but a pain in the butt
item when buying via internet mail order.