Where would you find a list of excess zoo animals!? It would be interesting to see what animals and where are trying to rehome!?
Zoo organisations have always produced Animal Surplus & Wanted lists but they aren't normally available for public scrutiny.
I know that the information is available through ZIMS but the only people I know who have access to ZIMS are zoo personnel as Pertinax has stated. I've seen ZIMS and it does indeed list surplus animals at collections - but a database is only ever as good as the information entered, it is fun doing a "virtual shopping list" though
The only way to see those lists is essentially to be a zoo worker with access to ZIMS or have a zoo keeper friend that will let you have a peek! Sometimes animals are advertised elsewhere but generally ZIMS is the first point of call.