The bulk of federal regulations pertaining to immigration reside in Title 8 of the CFR, entitled "Aliens & Nationality." These regulations can be found in the CFR databases of either the LexisNexis or Westlaw. In addition, up-to-date versions of Title 8 of the CFR can be found on the GPO FDsys website.
Often, legislation and regulations are explained through agency policy memorandum. Finding such policy statements can be helpful in immigration research or practice.