Ruger: http://shopruger.com/Revolvers/products/7400/
Brownells, or Gunparts,
Midway, $10 cheaper but back ordered: https://www.midwayusa.com/s?userSearchQuery=ruger+EJECTOR+HOUSING&userItemsPerPage=48
Don't settle for Borshardt, you won't be happy with the bluing color or the fit.