Hello,

Based on my information, the corruption index is published annually by the Transparency International since 1995. The organization itself was established in 1993. I think the data before that could be calculated and/ or estimated based on governmental data.

Good luck with your research!

Just look here:

Coppedge, M.J. et al.: V-Dem [Country-Year/Country-Date] Dataset v.5. Varieties of Democracy (V-Dem) Project.

Internet: www.v-dem.net/en/data/

There are data from 1900 onwards.