GDC (https://gdc-portal.nci.nih.gov/) has breast cancer data from TCGA. You can get the data from there. I think the metadata file of the sample or the clinical data from the breast cancer data have some data whether the sample is ER+ or not. This should help you identify the subset of samples you are interested in. Alternatively you can also search the GEO (https://www.ncbi.nlm.nih.gov/geo/) for a ER+ breast cancer dataset. Hope that helps, Cheers!