Hexblade, Sohei & Crusader give Mettle, as well as a few other prestige classes.

If you already have Mettle, the Tabard of Valor from Complete Champion gives you Improved Mettle if your HP are 1/2 your max or lower.

The Hexblade is the canonical example, though the Crusader also gets it. Or just look it up on Lists of Stuff - no point in reinventing the wheel...

If you've got incredibly high saving throws, then there's a feat or two in Player's Handbook 2 which stop you from auto-failing saves on a natural 1. So that's something.

As for dispel checks, Wizard 3/Master Specialist 10/Archmage 5 can theoretically achieve a dispel check bonus of +28.

I'm pretty sure you can do even better than that. There's a whole mini-handbook dedicated to it.

Pride domain for the win!

Tons of spells that grant you physical immunities.
The Iron Body spell gets you a lot of immunities, though it has some significant downsides.
Spell Compendium has several similar spells that transform your body to something elemental or undead and grant you appropriate resistances and immunities (Body of War, Elemental Body, Veil of Undeath).

I also often used Contingency as a defensive spell that would pop a Resiliant Sphere on my mage under appropriate conditions.
That brings up a really good defense, "not being targetable." No line of sight/effect (fog/resilient sphere), not being able to be sensed by magic (non-detection/Mind Blank), and full on not being there (fighting via proxy with Eidolons, summoned/called monsters, Simulacra, Cohorts, mind flunkies etc).

Personally, I feel resistances/immunities are best set up before the fight. Once initiative is rolled and action economy comes into play, the best defense is defeating the other guy. Lol, that reminds me of Suicide Sam Orc barbarian that rolled out with a negative AC but hit like a truck, made of tungsten, dropped from orbit.

The best defense is sending the party barbarian ahead of you!

Sooo... almost all I see here are spells or other magical effects. Anything the poor maligned martials can do to defend themselves? Outside of AC and HP I mean.

Send in the wizard first.

Almost everything interesting in D&D is a magical effect. Martial characters do get plenty of counters (not passive) and resistances to things - Paladins get immunity to fear, Hexblades get Mettle, Fighters have some ACFs, etc - but these mostly boil down to AC, saves, and immunity; you don't get Invisibility, flight, Mirror Image, miss chances (beyond perhaps a few oddballs), etc as these are almost inherently magical.