Is there a item for my monk that should have a high dexterity for my monk?

gauntlets of dexterity i believe though i dont remember where to find them I'm afraid. hope that pointed you in the right direction anyways

i think they're in the area with all the undead when you go down into the Unseeing Eye's chambers

Yes, deathcow is right, you can find those gauntlets in the ghoul lord area, but if i may give you some advice, and if you are still in the beginning of the game, i would make a new monk and make sure he has 18 str, dex and con, and then distribute your points in the other attributes, cause you will some very nice gauntlets in later stages of the game, so it would be a pety to waste your gauntlets slot on the gauntlets of dexterity.

EVERYONE WHO ROLLS A CHARACTER SHOULD HAVE 16 DEXTERITY!!! ALWAYS!!

you get significant bonuses to missile weapons, AC, and thieving skills. since AC is such a problem for monks, i would agree with Lord Delarue about making another monk. if you do that, you can give the gauntlets of dexterity to Keldorn, who you should have in your party when doing the Unseeing Eye anyway. Monks ought to have Keldorn in their parties anyway, since he can use a really nice 2hsword. he also can use the Heavy Crossbow of Accuracy you can get off Gaal, and later the Firetooth from the Watcher's Keep, in addition to two nice bastard swords there too.