Delegated legislation (aka secondary legislation)
In the words of Halsbury's Laws of England "... delegated legislation [can be] made by the Crown, government departments, local authorities or other bodies on whom legislative powers have been conferred by Act of Parliament, which are issued in various forms such as Orders in Council, rules, regulations, warrants, schemes, directions, and byelaws ... Many ... are termed Statutory Instruments." See OSCOLA 2.5 for how to cite SIs.
Halsbury's suggests Bradley and Ewing Constitutional and Administrative Law "For a discussion of the nomenclature of delegated legislation, statutory instruments, and subordinate legislation generally."
Halsbury's Statutory Instruments at KZ3 is the practitioners' tool in print. It has index and citator volumes - all of them regularly updated. Holders of an Oxford SSO can also access this service online via the LexisLibrary subscription.
Holders of an Oxford SSO.
The US materials in Westlaw US includes special collection called Regulations which includes Code of Federal Regulations (CFR), State collection, State Surveys and Regulation Tracking.
Sites on the free web
The Federal Register is the official daily publication for (inter alia) "proposed, interim, and final rules and regulations"
For more on US legal resources, please use guide below