While doing the 12 Ides 1000 Insanity series, I found that the only thing in the town where I had to start/finish in was the post office. I took a picture of my bike in front of it and posted it to Spotwalla, as well as including it in my app. I then rode to the nearest place where I could get a receipt (which in my case, was about 100 miles away!).
So in this case, if nothing is open or can give you a receipt, take a picture of your bike at some landmark and then go to the nearest place to get that Almighty DBR.
Also: ATMs are always, in my experience, reliable, and I have used them a few times when nothing else was around.
There is a gas station at the store on the east side off the road if you go almost to the end of 23.
IIRC Hartt's Exxon 42813 Highway 23 Venice, LA
I hope thats the one I used. I went all the way down 23 then stopped at the first one I came to.