what is it and where is it. prefferably not a robe. thanks

Well, for spellcasting it would be Robe Of Vecna, which is sold by Deirde in Adventurer's Mart.
But if you're planning to do something else with him, which I'm sure you will, seeing he's fighter/mage, go for Asylufreund Chain.
Said armor can only be got in ToB, by upgrading the armor you get from the dragon in Suldanesselar.
In SoA, it's probably the same one you get from that dragon.

In SoA it would indeed be that armor as far as I'm aware. Although Multi-class mages can wear any of the elven chainmails as far as I'm aware.

They still allow spell casting.

The earliest one that I can think of that you can get is from the Lord in the Mantle of Trademeet quest or whatever it's called. You can just go in and kill him with no rep loss.

Stoneskin makes up for any armours for figter/mages :p

Unmodded, the Bladesinger Chain is the best option - you get it in Chapter 7 when you kill the dragon in Suldanesselar. In ToB, with the upgraded armour and Protection from Magical Weapons (and the Amulet of Power), Solaufein can instantly make himself untouchable by any weapons for four rounds. Before that, your best bet is either the Robe of the Good Archmagi in Trademeet or Bracers of AC3 from the Adventurer's Mart.

If you have the bonus merchants, the Robe of Vecna is exceptionally good, unless you have a spellcaaster who needs it more.

Why do you need Armor? Spirit Armor + Stoneskin + Mirror Image is better than any Full Plate armor your other tanks could find...

Dispel Magic?

^ - Spell Immunity : Abjuration?

Touche :)
So, basically, F/M doesn't even need armor, but personally I prefer giving all my characters one, for those "when all fails" situations. Althought that is rare, you're in troubles when your F/M ends figting against horde of enemies, his protections just about to drop and there's no time to cast new ones. In that case, armor gives him some hope, actually it gives him much more hope than clothes.

That also takes up quite a bit of the battle... casting four spells just for protection. Someone could come up and beat Sola down in that time, or at least disrupt his concentration. Armour is quick and easy, and you can then spend your time blasting the enemies away instead of protecting yourself at the outset of each battle.

Minor Spell Sequencer - Blur and Mirror Image,
SI with Spell Shield, Stoneskin. If he's high enough level, use Spell Sequencer, too.

i'll end up with the bladesinger in think. thanks for the advice. :)

*Eventually* you'll end up with the Bladesinger, but until then the Elven Chain available via Trademeet (see starwalker's post above), or the Elven Chain +1 (siding with Bodhi in Chapter 3) are your earlier options for when Solaufein first joins.

P.S. Faraaz, point taken. ;) It's a theoretical point only though. SI: Abjuration is a waste of precious casting time and potentially a spell slot too, unless you use SI an awful lot. It's also simply a bad idea anyway ... "immunity" to Remove/Resist Fear, Free Action, and all Protection spells (including Prot. from Evil) ... No thanks.

SI: Abjuration is a waste of precious casting time and potentially a spell slot too, unless you use SI an awful lot. It's also simply a bad idea anyway ... "immunity" to Remove/Resist Fear, Free Action, and all Protection spells (including Prot. from Evil) ... No thanks.Yeah... it SHOULD do all of that, and in a PnP game, it would. Problem is, it doesn't if you cast it last (IIRC). It doesn't even remove other existing abjuration buffs (as it arguably should). It makes you immune to Breach and most spell-protection-removal spells, Dispel Magic, Remove Magic and Imprisonment.