Avinor uses PDC as the slot coordinator, and I think they are the main provider in Europe.
To simplify getting slot data from PDC Score (the system) we have defined the following services: Airport Slot Data Service v1.0 and Airport Slot Data Service v2.0. Version 2 is REST based and include operations for GA slot data consistent with the IATA GCR specification (slot-request-gcr-format-update.pdf).
The data sets defined are: