Just Google "68 Firebird Tail light socket" and you'll get hundreds of hits. And they can be found a local auto parts stores too...they are a pretty universal part. Roughly $3 to $8 each depending on where you buy.
I agree the metal ones like shown above would be a better choice for functionality, if being perfectly original is not critical.
Is there any difference between the inner and outer taillight socket? I wanted to go by the local autoparts store and don't have a chance to go take a look at my 69 firebird vert 350 to see if all 4 taillight sockets are the same
