Is it possible to get an ISSN number for an online journal from an other country?
Check the website/contact info with the specific agency, since I think this varies a lot. If I'm not mistaken, we got a Brazilian ISSN for a publication of papers resulting from the undegrad research program in my uni and it was all done by mail...

You can find the info for the agencies here http://www.issn.org/services/requesting-an-issn/