Ask your ISP for a ntp reference list. This is a service that all ISP's should be providing (IMO).
Many ISP's supply NTP on their DNS servers or their routers, but it sometimes takes a bit of digging to find someone who'll own up to its being there. Ideally the NTP server should be supplied by DHCP and it should an appropriate DNS name (eg. ntp.myisp.com), but few ISP's do this.
