Added Unbreakable Fighter 13.
Not sure if I should list this as a Stalwart ability, but Focused Dragon Expertise
grants Stalwart as an Inquisitor vs a dragon's extraordinary and supernatural abilities only, if you already have Evasion. With the Instant Enemy
spell you can make anything count as one of your types of favored enemy (you choose) "for all purposes" explicitly, so there is that for Rangers, but it still seems awfully limited and circumstantial.
There is also the Mesmerist's Astounding Avoidance
trick (works well with the Reflexive Trick
feat), but it only gives basically the exact same function as evasion - and at level 12, improved evasion - for all 3 saves. Meaning, it's only good for damage avoidance, rather than avoiding partial effects.
And lastly, I should mention the Charger
animal companion archetype, which gives your animal companion
(intended for mounts, but can be applied to any animal companion) a proper Stalwart ability, at level 15.