If a runway is "interesting", one can obtain the centreline longitudinal section elevations from the controlling airport organisation.
In days of old for Australia, when the good old paternalistic DCA looked after aerodromes and everything else to do with aviation, this information was provided routinely to those of us who played with performance work - at least for runways having a Type A chart published.
While the mean slope is fine for reasonably uniform runways, it is a very crude and brutish solution for those with any significant centreline variations .. especially for ASDR calculations if such end up being at all critically limiting. However, overall, that approach has served the Industry reasonably well over the years.