What sort of 'short notice' are we talking about here? Minutes, hours, days?
Because it stands to reason that the shorter the notice, the larger your 'pilot pool' is going to have to be. Which means 'network, network, network'.
Otherwise, to guarantee pilot availability, it sounds like you might have to pay someone a retainer. Then you would effectively have priority over their services.
But sorry if this sounds like an 'egg sucking class'!
I'm sure you've probably covered these options already.