My grandfather (now deceased) fought in the war, and I've been able to find in his old papers that he was with the Battery B 138th AAA Gun Battalion, however I CANNOT for the life of me figure out the larger division, etc. that that group was under. His personal records were lost to the '73 fire, so the only hope I have of getting an idea where he might've gone through the war is to track his unit's movements. Any help at all for where to look? searching for just the info I have turns up little. I think I need to be able to search more broadly. Thanks in advance, folks!
First of all, which country did he fight for?
I have a sneaking suspicion you are referring to the United States? At any rate, Orders of Battle can be found in most official or regimental histories. It might take some contextualizing, but official histories are very helpful for operational details and discovering which units/regiments/battalions form battle groups. A good place to begin is the Army Lineage Series Part 2 that explores field artillery. It can be found here.
EDIT: Sorry, Part 1 is what you need for 138th (see page 168) here.