The ISTD Dataset is proposed in Stacked Conditional Generative Adversarial Networks for Jointly Learning Shadow Detection and Shadow Removal, Jifeng Wang, Xiang Li and Jian Yang, CVPR 2018. Download from the the Github repository.
The ISTD dataset has some color difference in the non-shadow area of the shadow/shadow-free pairs so some color adjustment is needed. The code used here is provided by the authors of Shadow Removal via Shadow Image Decomposition, Hieu Le and Dimitris Samaras, ICCV 2019 (using MATLAB). Download from their project website.