
Rynjin |

Great Fortitude, obviously. There's several Traits that give a +1 as well.
There are actually quite a few magic items that outright negate certain prominent spells that go for the Fort save. Necklace of Adapatation makes you immune to Stinking Cloud, Cloudkill, and the like. Amulet of Proof Against Petrfication makes you immune to Flesh to Stone. Bead of Newt Prevention does likewise for a casting of Baleful polymrph (a steal at only 1000 gp too, though one use only).

tonyz |

Well, there are two ways to do this.
One is boost your Fort save directly: raise Con, add various bonuses (cloak of resistance to start with), take feats (Great Fortitude). Being a dwarf helps (Con bonus, some racial abilities).
Second is to do things that reduce the chance of you being targeted (disguises, invisibility so they can't see you,kill the spellcasters first, etc.)
At high levels you can do things like spell immunity and greater spell immunity, or even wishes to make yourself immune to particularly annoying types of spells.

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Multiclassing can help quite a bit.
For a melee investigator - a one or two level dip into Snakebit Striker Brawler wouldn't be bad. (Not necessarily the best solution - but a solution.)
Frankly - from 3.5 to PF, I think that save or suck spells got better. Not directly - but -
1 - Base races get bonuses to mental stats - raising the DCs. In 3.5 just about the only +0 LA races with bonus mental stats were a couple of the weird/broken elf subraces.
2 - Multiclassing. When you multiclass your saves go up - and 3.5's system promoted a lot more multiclassing. Therefore character's base saves were higher.

Paladin of Baha-who? |

What level? There's an investigator talent available at 9th that lets you spend uses of inspiration to boost saving throws. You can actually spend two uses to boost a saving throw even without that, but that's kind of annoying.
The Iron Guts rogue talent is an option if you absolutely must boost fort saves and have used all other avenues.
Ioun stones are available in a variety of types, many of which provide bonuses to saves or immunity to certain effects.