Last minute.. read less than 1–24 hours prior to flight time is a hazard zone for buyers. Airlines often block sales in the last 24 hours as they try to consolidate channels slow booking processes. Even there own. So they may make you buy at the counter.. which is another whole topic.
Airlines web sites may not sell you their flights as the flight has closed for sales 1–24 hours prior to flight time. This is a technology policy issue by the airline.
Meta engines like Mormomdo, Skycanner, or Wego are using cached data to show you flights that might be available via agents. The cache could be more than 24 hours old and will still be seen as offered. You could spend valuable time to only find out the booking is not able to be issued as the agent is locked out from booking due to time restrictions. OTA’s normally block flight purchases to the next day departures and beyond for security reasons and to avoid the airline lockout.
At Future.Travel - best travel website for airline tickets with no booking fee, we use realtime access to the airlines booking systems, so in some instances our gateway gives access to seats that airline check-in staff and cached websites can not access.
So the short answer for best pricing for last minute ticketing is to go direct to an OTA that has direct LIVE access to inventory (not a meta engine who shunts you off to who knows where) to see if the flight still appears, like we do on Future.Travel. If that does not work, try an airline that flys the route that you need to get to your destination. If all else fails, head to the airport and try to buy one over the counter. Some airlines offer standby fares at the counter.