There are many different ways to locate all the relevant statutory law you are concerned with. Here are some:
Once you identify a relevant code section, be sure to look at the surrounding sections. Generally, codification follows a certain format: purpose; definitions; application of the law; exceptions. Codified statutes are meant to be read and interpreted as part of the code as a whole.
As with any authority, you should use a citator to confirm that any statute you are analyzing or citing remains good law. Commercial citators such as Shepard's (Lexis) and KeyCite (Westlaw) will flag statutes that could be impacted by pending legislation.
Public Law: Public Law XX-XXX = P.L. [year of Congress] – [number of the bill – assigned in chronological order]
The law as published once the President signs it. In physical form, this is a soft bound booklet, depending on the size of the act. In electronic form, it is a standalone PDF.
Session Law: XX Statutes at Large XXXX = [Volume] Stat. [Page – assigned in chronological order]
The exact same thing as the Public Law, only given new numbering and bound with the other public laws, in chronological order, into a larger volume. In electronic form, it is part of a larger document.
Codified Version: XX United States Code XXX = [Volume] U.S.C. [Section]
The portions of law that are permanent (e.g., not the budget) are then codified by topic. (You can see the general topics in the Titles of the United States Code here.) This may involve creating new sections of the code to contain them, or fitting the language from them into previously established sections. Often, if there is a previously established section, the act itself will set out the changes to the codified version.
Sometimes codification can lead to one Act being broken up into different parts of a code. One common example is when an Act sets out certain behavior, and declares that not following that behavior can be charged as a crime. The portion related to the crime will be codified in Title 18, while the other portions will fall into whatever Title is most relevant –Copyright, Indians, Telecommunications, etc.
The U.S. Government Publishing Office provides authenticated versions of Federal statutes on its website, GovInfo.gov.
While the official sources are very important --and the sources to which you will often cite-- the annotated versions of the code, provided by different vendors, are often very useful. Look for each code's table of contents and index for more navigational options.
Popular Name tables help researchers quickly find session laws and codified statutes by their commonly-used names.
States produce statutes in session law and codified format. The names used for session laws and statutory codes will vary from state to state.
For example, in Pennsylvania, you have the Laws of Pennsylvania, or the Unconsolidated Statutes (the session laws), and the Consolidated Statutes (the codification). But the official-ness of the Consolidated Statutes is not entirely complete, nor is the codification. You can read more about this here. You can get an even better idea of how complicated states and official codes can be by taking a look at this 2011 survey from the National Conference of State Legislatures.
There are several publishers that specialize in municipal laws and regulations; some of the best-known are linked below. Searching the name of the city + code (or type of code, such as zoning code) can often get you to the resource you need.