What unique Blades are not available through random drop from generic Cores Crystals?
Solution 1:
For the sake of completion, I'm going to include the ones you mentioned. There are 8 in total.
Roc - Roc's Core Crystal - gained via story
Wulfric - Beastly Core Crystal - gained via story
Kasandra - Lucky Core Crystal - gained via salvaging at the salvage point in Chansagh Wastes until a Gluttonous Marrin spawns, which would drop this crystal
Herald - Sealed Core Crystal - gained via feeding "blood-related" items to the Shrine of Sealing in Theosoir, Tantal
Sheba - Inherited Core Crystal - gained via spending 500k at the shop in Gormott
Vess - Vess's Core Crystal - gained by doing Vess' sidequest ("Tranquility") in Gormott
Praxis - Praxis's Core Crystal - gained by doing the Crystal Clear side quest
Theory - Theory's Core Crystal - gained by doing the Blade-Sharp Memory side quest (this is related to Praxis)
In fact, one thing worth mentioning is that all of these Blades are available through a common core crystal drop if you release them. This is a strategy that was used to trade Blades before Overdrive Protocol farming was possible.