Using a rubber or nylon o-ring from the hardware store isn't the right part. They need to be made of viton.
If you are asking about the small o-rings ontop of the intake(rear by distibutor) I just bought some from g.m last week
#;s were:
22514722- Seal o-ring $1.48
22516256 Seal $1.25
I just bought these O-rings too, and yes they are available still from the stealership. I wouldnt use anything else.