Due to the sensitive nature of APT and that it is closely aligned to espionage, the only real way to get a suitable feed will be through Government or National Law Enforcement agencies.
The difficulty will be in establishing a level of trust to enable the sharing of information.
For organisations within the USA, the advice is to contact your local FBI field office and arrange for a threat briefing on the subject.
For organisations within the UK, then making contact with CPNI (Centre for the Protection of National Infrastructure) would be the best option. CPNI hold a number of industry specific 'Information Exchange' forums that may be of help. However it is worth noting that CPNI has a focus on the critical national infrastructure for the UK and if your organisation falls outside of this may impact on access to that.