Ideally, subciting involves finding PDF copies of many types of materials. PDF is an acceptable format for subciting because the scanned image maintains the pagination of the original document, thereby ensuring perfect harmony in the world according to the Bluebook. (I kid, but frankly I'm with the Bluebook on this one.) The single best resource for getting a PDF of law journals is HeinOnline.
Throughout this guide are links to HeinOnline for law review articles, for decisions in U.S. Reports, for the CFR, and for other frequently cited documents. Here are just a few of the sources you can find in citable format through HeinOnline (all from the comfort of your couch):