Is there an Assassin multiclass feat that grants Shade Form? I found one for Assassin's Shroud and Shadow Step but I can't find one for Shade Form.
There is one, but it's not useful:
Iff you paragon multiclass, Darkened Soul allows you to gain the class feature Shade Form.
There isn't one.
Pre-Essentials Assassins have very, very little feat support, including in the multiclass department. And even the core classes don't have multiclass feats to mimic all their class features.