Wizard spell DC

[Trandir]

Usually a wizard without any particular support get around DC 27 of a 9th level spell.

How can one increase it via feats, items, ACF ecc.. but without multiclassing or PrC?
I am looking at you tainted scholar

[Esprit15]

Besides the school specialization feats, illusionists get Shadow Weave Magic, Mastery of Dreams, and Shadow Trickster as options to buff their save DCs. Vow of Nonviolence is a potential option, but only in certain campaigns.

[Gauntlet]

Master Specialist has several DC buffs available, both in the GSF bonus feat and in the school esoterica.

[Oberron]

Tainted spell casting 8s another way but a lot of people don't use taint rules

[Anthrowhale]

Earth Spell gives you another +1.

[Biggus]

There's a feat in Libris Mortis called Mother Cyst (p.28) which gives you access to a spell called Necrotic Empowerment (p.68) which gives you a +8 enhancement bonus to Intelligence (among other things) for short periods. If you're undead, the feat Fell Energy Spell (Dragon Compendium) can increase this by a further +2.

There are a handful of ways to get non-enhancement bonuses to Intelligence: have a look at this (http://minmaxforum.com/index.php?topic=176.0) list.

EDIT: also, in Frostburn there are feats called Cold Focus and Greater Cold Focus which increase the DC of spells with the cold descriptor by 1 each.