Here are some public newsservers (ie, ones which anyone can use). You should only need these if your ISP doesn't have its own newsserver.
gate.or-computers.de
news.ausmail.com
news.okidata.com
news.spar.ca
news.tin.it
news.uov.net
news-reader.wcg.net
nnrp.news.uk.psi.net
piggy.rz.tu-ilmenau.de
pub.news.uk.psi.net
sion1.bathspa.ac.uk
Note that not all newsservers allow posting - some are read only. You will be informed if this is the case when trying to post to such a newsserver.
Also, try these websites for information on public newsservers:
http://www.newzbot.com
http://www-stud.uni-essen.de/~se0067/news.html