Are the four special issue heroes available anywhere for purchase.

Harvey

The last time they were available for purchase was during Christmas. You can still get Nara the Fang possibly if you find a first print copy of SoB. There will be a nice round sticker telling you he's in there. The other three were offered for sale on FFG's website, but they sold FAST. Hopefully there will be another round of sales since the figures were reprinted for the Realms of Terrinoth event where the promo figures got new cards to allow their play in Runewars. Anything could happen really.

+1 to Proto.

Also, I'd be surprised if all the first print run copies of SoB are gone this fast, so hopefully you shouldn't have trouble finding SoB with Nara inside. I don't really know how long it takes for a first print run to run out (I suppose it depends on how well it sells), but I'd expect to get at least a year or two out of an initial, non-limited print run of most products.