They're really not that scattered. Imperial Armour Apocalypse Second Edition has all of the non-Apocalypse units (all 3 of them). If you're playing Apocalypse, chances are you will have the Apocalypse book anyway, so it's not an extra purchase getting the rules for the Heirophant and Barbed Heirodule. The only real extra purchase is Imperial Armour Aeronautica for the Harridan, though that's hardly needed. You can get by fine with the Harridan rules in IAA Second Edition.
As mentioned though, the non-Apocalypse units aren't too hot. The Malanthrope is kind of neat, granting preferred enemy for any unit they kill to any Tyranid unit within synapse range, though it's an ability that won't come up very often, and even then, won't make much of an effect. They are also the only synapse unit to not have Shadow in the Warp, which is really dumb. Meiotic spores are basically super spore mines. The Stone Crusher is a Carnifex with a single set of Crushing Claws, a 2+ save, slightly better regeneration and bonuses on the building damage table. There's not even any upgrade rules for having the Wrecker Ball, and can't be brooded. I think they have Fleet, but can't quite recall. Though I do give
FW credit for actually making rules for Stone Crushers, previously they were just intended for Carnifexes with the Wrecker strategem.
Of the Apocalypse units, only the Harridan and Heirophant are really worth it. They're both super tough and dish out plenty of hurt, though will become a D strength weapon magnet quickly. The Heirodules really con't need to stay confined to Apocalypse anymore - their stats and model size are comparable to the Tyrannofex and Tervigon. Simply changing them to be monstrous creatures, changing the Scythed Heirodule to have the Tyrannofexes Pyro Acid spray and the Barbed Heirodule to maybe two rupture cannons or one twin linked rupture cannon would slot them nicely into standard
40k.