Since Powell's has a huge used book inventory, I decided to look for inexpensive editions from these authors:
Patricia Moyes: this is a relatively recent author who writes mysteries in the "golden age" style. Her books are available on kindle, but they are a bit pricey for books of their age.
Madeleine Brent: these aren't available in ebook at all, and the used copies are fairly expensive. I am not sure if I'll be able to get my hands on any of them, but I'll give it a look-see.
Patricia Highsmith: I am on an "I want to read more Highsmith" kick, so I'll just take a look and see what they have!
Agatha Christie: anything that I don't already own in print is fair game!
Ngaio Marsh: these are also available in ebook, for a fairly reasonable price, so I'll only pick the up if they're under $4.00.
John Dickson Carr: he is pretty unavailable, so I'm going to take a look see!
I may come up with more as I think it over today! If anyone has any suggestion for OOP gems, especially mysteries, let me know in the comments!