ISO/IEC standards can be purchased from ISO and usually from your local national body. The lack of free online availability has effectively made ISO standard irrelevant to the (home/hacker section of the) Open Source community. However, many important ISO standards can be located and downloaded for free legally if you know where to look.
The first source is ISO itself. Where an ISO standard is based on a pre-existing external specification that is itself freely available, (a “Publicly Available Specification” in ISO-speak), the committee managing the standardizing process can ask ISO to make it available on the Publicly Available Standards webpage. This typically is used for standards that come in from an external boutique standards body, but can come from companies (e.g. MS C#) or even from individuals (as was the case with standardizing Schematron.)
These standards do have some encumbrances: you have a single user license and you may only retain one printed copy. This is perfectly adequate for a single open source developer or student.
The ISO list contains the standards for programming languages (FORTRAN, C, BNF, ECMAScript, C#, CLI, Eiffel, Ada profile), graphics (CGM), networking and data interchange (OSI, X.25, parts of EDI, phone systems, the basis of Unicode), data formats (ASN.1, parts of MPEG, .iso CD format, JPEG2000, ODF), hardware (data cartriges, optical disks), and even the Linux application binary interface (a near subset of ISO POSIX), some of current and some of historical interest, and many standards establishing common vocabularies and on conformance testing. Most important for XML people, it has the growing list ISO DSDL schemas languages (RELAX NG, Schematron, etc). It also includes some Technical Reports, which are not standards but more like backgrounders or tutorials: ISO/IEC TR 15285:1998 Information technology — An operational model for characters and glyphs is a good example.
What is notably missing? The subsets of PDF for pre-press exchange (PDF/X) and for archiving (PDF/A) would be nice.
National Bodies and Industrial Consortia
When an ISO standard is a rubberstamp of a national standard or industry consortium specification, the original is frequently available from the original site. For example, OASIS, Ecma. Sometimes a standard is augmented with extra information that becomes the preferred distribution: this is the case with Unicode Consortium’s augmentation of ISO 10646 as the Unicode Character Set.
Now the other source of standards material are draft versions. As a standard develops through a committee, there are very often discussion drafts made, and these frequently make their way onto the internet, to help discussion and promotion and to provide a record of the progress. Don’t copy them. International standards have an IS number, and technical reports have a TR number.
he early drafts of a standard are called Committee Drafts and have a CD number: you should be very wary of these.
The versions that make it to an initial vote are called Draft International Standards and have a DIS number; these are usually pretty good indications of the final standard so that there may be a few changes every few pages, they are good enough that the Steering Committee (SC) at ISO presented them for an international ballot.
A draft standard that has had all the changes made from the DIS ballot and is submitted for a final vote is the Final Draft International Standard with an FDIS number; these are gold: rare and valuable, because even though you can expect the final standard to only differ in small editorial ways (typos fixed, etc) usually committee members take them off public websites when the IS is published at ISO. There can be a gap of many months between when an FDIS is accepted at ballot and when the book is available published from ISO, so the online version can help tide people over that period; typically, unless the standard is destined for the free list above, the FDIS would be taken off the website at that time, which ISO asks for.
These drafts online serve a very valuable purpose: they are very convenient for helping developers decide whether a standard might be useful for them. After deciding, of course, a serious developer would then progress to supporting ISO and buying the copy. While a CD is unsuitable for the purpose, a DIS is often good enough to use for prototyping some software, with the caveat that you should check whether there were a succession of DIS in case it was contentious.
How do you find these drafts? Google for “ISO” and the keyword for the technology you are looking for. Then look for numbers, preferably with a DIS or FDIS in them. You may also use the same method to find out which committee as ISO handled the standard (look for “TC” or “SC”)’; many of them have websites with all their formal material including drafts and comments. For example, SC34 is here thanks to Ken Holman’s efforts. An example of a draft archived on the committee site is the ISO C++ draft.
Another good source is Open Standards, which hosts the websites for many ISO groups, such as the POSIX effort.
For some extra sources for researching other standards, there is a helpful page from Ryerson University.