520 Spiral 
The location for the showdown between the Scoobies and the Knights of Byzantium is not an actual gas station, just a set used as a location for filming (shortly after visiting it, I saw it used in a commercial for Kaplan University). It's called Four Aces. If you tell them you're there to scouting locations for a shoot, they'll let you go inside. While some of the interior shots for "Spiral" may have been done on a set, many of them clearly are done at the location, particularly in the part right next to the pumps. Still, the interior is actually very different than the way it was in the episode. I haven't organized the photos yet but many of them are reproductions of shots from the episode.
My guess is that the very final shot of BtVS from Chosen was done on E Palmdale Avenue somewhere near here, but I'm not sure.
15 Photos (taken January 14, 2009)
Location: 14449 Avenue Q, Palmdale, at 145th St.
How to get there: Head north from LA on the 110. Get on I-5 and go north towards Sacramento for about 25 miles. Take Exit 162 and get on CA-14 heading north toward Palmdale. Go about 35 miles until you get to Palmdale. Take exit 35 onto W Palmdale Blvd/CA-138 and head east. Head through town. After about 11 miles, turn left (north) onto 110th St. (when I went there, there was detour due to construction but that should be done by now). After half a mile, turn right (east) onto Avenue Q, which is a dirt road. Right away, there's a little dogleg south. A mile along there is a corresponding little dogleg north which marks the beginning of the shooting location. The scenes with the attack by the Knights Who Say Key take place along this stretch of Avenue Q. The gas station where the rest of the episode was shot is about 3 1/2 miles east of 110th St at the intersection of Avenue Q & 145th St.
Mapquest here.
Google Earth Co-ordinates: 34 35' 15.65" N, 117 52' 18.86 W
