for finding part numbers, i often go with gmpartsdirect.com, but have found that their online catalog can be lacking for the 'small' stuffs... so a call to a dealer that'* local is always good - you'll find that of all the people at dealers, the parts guys are a breed all their own. i've never met one who wouldn't go out of his way to find a part number for a guy, and have even had them offer on many occasions to fax me drawings of a part or assembly so I could figure things out.
as for the p/n of those o-rings, i just went to advanced auto, and they had them there - i don't think autozone did... but i didn't want to get ripped at a dealer. even as a gm employee, i usually can only get them to give me the gm list price instead of the super inflated dealer price....
but anyhew, it was a felpro set for the l36 (assuming it would be somewhat simliar for the l67), but it was made for 4, 6, and 8 bangers, so each box has 4. meaning i had to buy two boxes, and have two o-rings left over. so it goes - they were only like 3 bucks a piece...
96 LeSabre (L36)
95 Olds LSS (L67)