The SAD1024 is winding down in terms of availability. People have sought out just about every NOS source that exists. I imagine there's still a TV/stereo repair guy on an island in the Philipines that has two in a bin, and another guy in an electronics shop in Montevideo that has changed owners 3 times who doesn't know what those 3 chips in the drawer are for. But that seems to be about it.
More recently, folks have been trying to retrofit SAD1024-based devices with the MN3007 or MN3207. Those chips can be made to accomplish the same things as an SAD1024, but it is unfortunately not as simple as taking the one chip out and plunking the other one in. You'd need to make a little daughter board with a convertor.
All of that being said, Steve Daniels at Small Bear Electronics gets in a wide variety of delay chips and is a fairly good source for these things. http://www.smallbearelec.com/StoreFront.bok
Although he and others (e.g., Analog Mike: http://www.analogman.com/parts.htm) appear to be out of 1024s.
I'm actually kind of curious about what MXR themselves is using to produce the reissue Flanger.