How to find a drop-in-replacement ADC
This is the bane of designers. There really isn't an easy way to get around it; you either design your boards such that they accept similar parts from different vendors, or you try to establish some kind of guarantee of availability with the manufacturers you do use.

Since most of us are not Apple or Sony or another huge volume company, you're stuck with the multiple vendors route. :-)

There really is no other way around it.  Personally I avoid companies such as Rochester for the very reason that they are a "last resort" type of company, generally with prices that match (i.e. they know they've got you by the short and curlies).

Memories and things like ethernet PHYs are pretty easy to make boards work with multiple vendors or with multiple parts from the same vendor; you usually end up with a few resistor "jumpers" to route the pins that aren't at the same location.  Other devices you usually end up with multiple footprints on the board, sometimes with creative overlapping of the footprints to minimize the amount of space they're taking up.

Sorry I don't have better news for you. In my own experience I have gotten such that I will have a spot on the BOM for "alternative part numbers" and some of my device footprints are already set up to accept them. You live and learn. :-)