Several of the hardback adventures identify important agents of the main factions. A quick google, however, has failed to turn up the identity of any outright heads/leaders of the factions. Is my google-fu weak or do we just lack details about this?
I think WotC might be intentionally vague, so that DMs can further develop their own leaders. I do know more fairly canonical information may be found at both the Forgotten Realms Wiki and at Candlekeep. However, it bears to keep in mind that Candlekeep tends to follow any writings of Mr. Ed Greenwood, the originator of the Forgotten Realms, in preference to anything said by WotC (though both will likely be mentioned and source attributed, whereas the FR wiki tends to collate all WotC official info.
EDIT: Unfortunately, I cannot seem to find any search engines on the Candlekeep website ( www.candlekeep.com ) but if you go to Google, and search for specific terms (e.g. Harpers) and add "site:candlekeep.com" that should help. However, just doing so I don't note a whole lot of good exposition there other than from a few posts in their forums.
I think the 5E Forgotten Realms as set up are fairly localized: the neighborhood Zhentarim ringleader is more important than somebody out by the Moonsea for any given location. The Order of the Gauntlet has it's own country, but that hasn't been heavily detailed (yet...?).