ok Sorry about not giving enough information on this. the games is BGTOS. my charecter is fine on his armor and the rest of my party is fine other then the first necromancer that joins your party after gorion dies when you start exploring with imoan. he has decent spells and wepons but his ac is 8 were has the rest of my party is 0 to the negatives. this is my first play through with him and his friend in my party but i can't remember where you can get anything good before you go into the mines to fix the iron shortage. the reason i asked for the codes was in case you can't get him any decant armor till after than mines.

It'd help if you specified which particular game you're talking about (BG, BG2, BGTutu, BGT).
That said, Rings/Cloaks/Amulets of Protection and Robes of the Good/Neutral/Evil Archmagi are available in all versions of the game, and you should be able to learn spells to either directly improve your AC or otherwise prevent damage from attacks (Armor, Shield, Mirror Image, Stoneskin).

Also, there are plenty of potions (armor, shield, etc) you can keep on him to give him the AC boost when he needs it. I'd recommend doing all you can to keep him out of combat unless it's critical or an ambush. Then down a potion and start some scary necromancing.

What game are you playing? We can't give you any specific info until we know that.

Are you soloing? If not, your necromancer shouldn't need AC, because he shouldn't be targetted in the first place. Leave the tanking to the fighter-types.
Mages are not supposed to have high AC, since they can't equip armor in the first place. Unless of course you made a fighter/mage or cleric/mage multiclass, in which case you are likely the best tank in your party. Even then, spells like mirror image, blur, shield and stone skin plus whatever bracer or robe gives you the best AC is typically your better option.

If you're thinking about Xzar then you could consider the Robe of the Evil Archmagi (AC 5) and the things Shadowsetzer mentioned. Thalantyr in High Edge sells the Claw of Kazgaroth which adds one point but comes with a penalty to CON and that is not a problem for Xzar since it will lower his CON from 10 to 8 (and HP penalty begins at 6) you should consider it for a Necromancer with either 18 points in CON or a score ranging from 14 to 9 since it won't harm the character (there is however a penalty to Death saves but it is compensated by bonuses to other saves, just make sure you're carrying some antidote if the character gets poisoned).

If it's your first time playing the game just keep exploring and you'll find better stuff.
Just take the best sling you find, buy some bullets and shoot from the distance, you play a caster, not a fighter. In the beginning you have only 1-2 spells per day so you have to rely on party members for survival.

If the game is so hard that you want to cheat in equipment consider changing your character stats instead. 18 dex and 16-18 con help surviving a lot.