Monte Cook's Arcana Unearthed/Evolved has an item that grants +1 DC to your spells for 12,000gp cost.
If the save DC issue is really getting you down, I suggest a build something like this using the Elemental Savant PRC from Complete Arcane:
Human Sorcerer (Cold Evocation Specialist)
Lev 1 Sorc Spell Focus feat: Evocations +1 DC, Greater Spell Focus feat: Evocations +1 DC, Familiar
Lev 2 Sorc
Lev 3 Sorc Energy Substitution feat: Cold
Lev 4 Sorc
Lev 5 Sorc
Lev 6 Elemental Savant (Cold), Resistance to Cold 5, Elemental Specialty (all energy spells auto-convert to cold), Draconic Heritage Feat (Complete Arcane): Silver (Disguise Class Skill, +2 v. Sleep Paralysis Cold)
Lev 7 ElmSv Immunity to sleep
Lev 8 ElmSv Energy Penetration: Cold (SRP +2)
Lev 9 ElmSv Restistance to cold 10, Draconic Power feat (Complete Arcane) (+1 Caster level and DC all cold spells)
Lev 10 ElmSv Energy Focus: Cold +1 (-1 Spell Casting Level)
Lev 11 ElmSv Darkvision
Lev 12 ElmSv Resistance to cold 20, Cold Focus Feat (from Frostburn) +1 DC
Lev 13 ElmSv Energy Penetration: Cold +4
Lev 14 ElmSv Immunity to Paralysis and Poison
Lev 15 ElmSv Elemental Perfection, Energy Focus: Cold +2, Energy Immunity: Cold, Greater Cold Focus Feat (from Frostburn) +1 DC (-1 Spell Casting Level)
+7 DC to cold evocations by 15th lev
+5 SRP to cold spells by 15th lev
+1 Caster Lev to cold spells by 9th lev
You might consider replacing a feat with Spell Thematics for an addition +1 caster level and +1 SRP to some cold spells. The Frost Mage PRC is also an option with this build (from Frostburn), perhaps for levels 16-20.
This build could also work well with a Wizard if you are allowed by your DM to use the Evoker Variants on page 62 of Unearthed Aracana. You would no longer get the two dragon feats above since you are not a sorceror, however the familar replacement and other variants make up for it and also free up a feat (and possibly give you an extra feat as well for your 5th level wizard bonus feat, if you don't replace it).