Grey-96

I have been trying to find the right combination of Engine and Transmission to swap into my 96 Roadmaster and still coming up blank after several years of looking. I'm doing something wrong or perhaps expecting too much of the used engine market. I'm a retired gearhead that cannot afford new stuff. So the logical thing to do, is build excitement into my old car. I want the GM L99 technology with an 8L90E transmission. That has been an option in GM vehicles since 2015. Why can I not find this combination in any savage yard. Too new yet? My thought is buying the drop out assembly with all the wiring and control modules as well as the "floor board" with the properly formed transmission tunnel. The old 1996 4L60e tunnel just ain't going to fit the 8L90E housing. Any suggestions on where to look?