I am an Emerald Aisle member and the search results told me that no cars are available at SeaTac for Wed, December 13. If I just show up, am I likely to get a car?

My experience is car are coming from the clean up facility constantly. I would take a chance and ask for the sister agency if you have to wait longer than 5 minutes. There is usually a representative by the aisle.

Probably not. But you can check sister company Alamo.