It may depend on exactly what you need.

The ISSN Centre offer licensed access to their ISSN portal at a cost http://www.issn.org - my experience is that this is pretty comprehensive
The ISSN Centre also offer a download of ISSN-L tables - this is available for free (although you have to state what you intend to do with it before you can download) - this is just ISSNs (mapped to their ISSN-Ls) but if you don't need bibliographic details then it would be a good source
As well as WorldCat you could also try Suncat which offers a z39.50 connection http://www.suncat.ac.uk/support/z-target.shtml, but obviously this has the same issue as the WorldCat approach
GOKb and KB+ are both initiatives trying to build knowledgebases containing many ISSNs with data to be made available under a CC0 declaration. Both of these are focussed on describing bundles/packages of journals. GOKb is going to be going into preview imminently (http://gokb.org/news) and KB+ already offers downloads http://www.kbplus.ac.uk/kbplus/publicExport. KB+ currently has details of around 25k journals.
There may also be some largescale open data initiatives that give you a reasonably good set of ISSNs. For example the RLUK release of 60m+ records at http://www.theeuropeanlibrary.org/tel4/access/data/lod, or the 12million records released by Harvard http://openmetadata.lib.harvard.edu/bibdata (both CC0)

Owen

Owen Stephens
Owen Stephens Consulting
Web: http://www.ostephens.com
Email: [log in to unmask]
Telephone: 0121 288 6936

On 17 Oct 2014, at 03:16, Stuart Yeates <[log in to unmask]> wrote:

> My understanding is that there is no universal ISSN list but that worldcat allows querying of their database by ISSN. 
> 
> Which method of sampling the ISSN namespace is going to cause least pain? http://www.worldcat.org/ISSN/ seems to be the one talked about, but is there another that's less resource intensive? Maybe someone's already exported this data?
> 
> cheers
> stuart
> --
> I have a new phone number: 04 463 5692