Yea, marathonguide.com is a good pace to look. I searched for puff pieces about spring marathons and Boston qualifiers, and then researched more at marathonguide.com.
North America actually has a surprisingly small number of marathons with good depth under the 2:40 range. Off the top of my head, you have the Majors (Boston, Chicago, NYC), Cali International, Houston, Grandmas, Ottawa, and Toronto Waterfront. What else is out there with at least a few dozen runners coming in under 2:40?
I’m biased, but I also find the Indianapolis Monumental to be surprisingly deep because it has a growing reputation for being a fast course and is a cheaper, lower hassle fall alternative to Chicago/NYC (31 runners under 2:40 in 2017).
I think the non-majors are attracting a growing number of fast runners because the cost and hassle of the Majors is becoming too much.