I'm totally new to ARs, so this might be a real dumb question.  I few months ago I ordered a lower, a lower parts kit and a stock from a local dealer.  Rather than the kit, it came assembled, with the stock sort of half screwed on.  I got my complete upper last week, and and noticed that neither came with the release pin, spring or detant.  It seems to me like these parts should have come with the lower.  The dealer is closed until Tuesday, but should I expect to get those parts from him, or do I need to hunt them down somewhere?  

Quoted: the release pin, spring or detant.

Are you refering to the rear takedown pin? If so, the spring and detent are held in by the stock itself. They should have been included with the lower receiver parts kit.

should be included in the lower parts kit

Quoted: should be included in the lower parts kit

+1 and I would hunt down the dealer that sold you the lower and get the parts from him!

Thanks.  It's the takedown pin, and associated parts that are missing.  I'll try the dealer on Tuesday.

Quoted: Thanks.  It's the takedown pin, and associated parts that are missing.  I'll try the dealer on Tuesday.

If you got a lower with LPK those parts should have been included.