D.C. Code of Municipal Regulations: The regulations promulgated by D.C. administrative agencies are codified in the District of Columbia Municipal Regulations, commonly known as the "DCMR." (Note: The DCMR also contains selected laws enacted by the D.C. Council.)
The D.C. Office of Documents and Administrative Issuances published the official DCMR in print until the end of 2009, after which only a few copies where produced for historical purposed. As of 2010, the official DCMR is the free online edition. Caution: The official DCMR is generally not updated promptly.
Lexis publishes an unofficial edition (formerly published by Weil's Codes) called the Code of D.C. Municipal Regulations, which is usually within a month or two of being current. You can search the Lexis/Weil's Code on Lexis (DC;DCADMIN). You can search the official code, updated by West, on Westlaw (DC-MR). The DCMR is also searchable on Loislaw.
Index: The official edition of the DCMR does not have an index. Instead, you can use the index in the Lexis/Weil's edition, the index in the DCMR Deskbook (R.T. Associates), or you can search an electronic edition of the DCMR.
Historical Editions: Westlaw has historical editions of the DCMR back to 2002 (e.g., DC-MR02 for 2002).
Proposed and Final Regulations: Proposed Regulations (other than Emergency Regulations) and all Final Regulations are published in the District of Columbia Register. The Office of the Secretary posts the Register online free from April 2003 to the present. The DC Regulations site has Register issues from October 2009 to the present. Note: General distribution of the print Register stopped at the end of 2009, though a few copies were still being published for archival purposes.
Most of the regulations in the Register are available from 1998 on Lexis (DC;DCRGST) and from 1999 on Westlaw (DC-ADR). You can get copies from older issues from the LA County Law Library's document delivery service, or call one of the D.C.-area law school libraries. Subscribers can get issues back to volume 35 (1986) from the Digital Session Laws collection on HeinOnline.
The Register is discussed further near the end of the "District of Columbia - Legislative Branch" entry.
Regulatory History: If you need additional background on a regulation, search the District of Columbia Register call the relevant agency to see if any regulatory history materials are available. You might also want to search a D.C. news database to see if the matter was covered in the press.
Commissioner's Orders: When D.C. was governed by Commissioners, regulations were issued as "Commissioner's Orders." Commissioner's Orders were published in the District of Columbia Register from July 1954, when the Register began publication, until 1969, when the Commissioners were replaced by the Mayor and City Council. According to the History of the Office of Documents, "to determine what the non-statutory law in the District was before 1954, it is most often necessary to engage in a difficult and very tedious search of the minutes of the meetings of the Commissioners, which are stored at the National Archives."
D.C. Rules and Regulations (DCRR): The DCRR was an early, incomplete attempt to codify D.C. regulations in the mid-1970s. Each title was published separately as a "special edition" of the D.C. Register. According to the D.C. Administrative Procedure Act, regulations not included in the DCRR were not valid, but I have read that this rule was not always followed.