For many questions the answer seems to be found in "the standard". However, where do we find that? Preferably online.
Googling can sometimes feel futile, again especially for the C standards, since they are drowned in the flood of discussions on programming forums.
To get this started, since these are the ones I am searching for right now, where are there good online resources for:
As of 1st September 2014, the best locations by price for C and C++ standards documents in PDF are:
C++98 – ISO/IEC 14882:1998: $90 NZD (about $60 US) from Standards New Zealand
C17/C18 – ISO/IEC 9899:2018: $185 from SAI Global / $116 from INCITS/ANSI / N2176 / c17_updated_proposed_fdis.pdf draft from November 2017 (Link broken, see Wayback Machine N2176)
You cannot usually get old revisions of a standard (any standard) directly from the standards bodies shortly after a new edition of the standard is released. Thus, standards for C89, C90, C99, C++98, C++03 will be hard to find for purchase from a standards body. If you need an old revision of a standard, check Techstreet as one possible source. For example, it can still provide the Canadian version CAN/CSA-ISO/IEC 9899:1990 standard in PDF, for a fee.
Print copies of the standards are available from national standards bodies and ISO but are very expensive.
If you want a hardcopy of the C90 standard for much less money than above, you may be able to find a cheap used copy of Herb Schildt's book The Annotated ANSI Standard at Amazon, which contains the actual text of the standard (useful) and commentary on the standard (less useful - it contains several dangerous and misleading errors).
The C99 and C++03 standards are available in book form from Wiley and the BSI (British Standards Institute):
The working drafts for future standards are often available from the committee websites:
If you want to get drafts from the current or earlier C/C++ standards, there are some available for free on the internet:
I cannot find a PDF of C89, but it is almost the same as the below draft for ISO/IEC 9899:1990 (C90). The only differences are in the boilerplate and section numbering.
https://www.pdf-archive.com/2014/10/02/ansi-iso-9899-1990-1/ansi-iso-9899-1990-1.pdf
(Almost the same as ANSI X3.159-198 (C89) except for the frontmatter and section numbering. Note that the conversion between ANSI and ISO/IEC Standard is seen inside this document, the document refers to its name as "ANSI/ISO: 9899/99" although this isn't the right name of the later made standard of it, the right name is "ISO/IEC 9899:1990")
http://www.lirmm.fr/~ducour/Doc-objets/ISO+IEC+14882-1998.pdf
https://cs.nyu.edu/courses/fall11/CSCI-GA.2110-003/documents/c++2003std.pdf
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf
https://github.com/cplusplus/draft/blob/master/papers/n4140.pdf?raw=true
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf
Note that these documents are not the same as the standard, though the versions just prior to the meetings that decide on a standard are usually very close to what is in the final standard. The FCD (Final Committee Draft) versions are password protected; you need to be on the standards committee to get them.
Even though the draft versions might be very close to the final ratified versions of the standards, some of this post's editors would strongly advise you to get a copy of the actual documents — especially if you're planning on quoting them as references. Of course, starving students should go ahead and use the drafts if strapped for cash.
It appears that, if you are willing and able to wait a few months after ratification of a standard, to search for "INCITS/ISO/IEC" instead of "ISO/IEC" when looking for a standard is the key. By doing so, one of this post's editors was able to find the C11 and C++11 standards at reasonable prices. For example, if you search for "INCITS/ISO/IEC 9899:2011" instead of "ISO/IEC 9899:2011" on webstore.ansi.org you will find the reasonably priced PDF version.
The site https://wg21.link/ provides short-URL links to the C++ current working draft and draft standards, and committee papers:
The current draft of the standard is maintained as LaTeX sources on Github. These sources can be converted to HTML using cxxdraft-htmlgen. The following sites maintain HTML pages so generated:
Tim Song also maintains generated HTML and PDF versions of the Networking TS and Ranges TS.