I need things like a timing chain, temp switch, oil pressure sending unit, motor mounts, things like that. Where is the best place to buy these things? Is there some classic Buick store? Some specialty classic car store?
You might want to mention what car you need parts for. Many basic items such as those mentioned can still be purchased from your local parts stores. Otherwise, TA Performance specializes in 60's/70's stuff, performance and stock items also.
I'm recommending classicbuicks.com at the moment. If OPGI ever gets it's catalog published for the big cars, they will be a serious contender.
