Few units of battalion strength formed veterans associations. Especially support units. Except for those which were organic elements of the armored divisions, tank battalions were assigned to field army or army corps (usually the latter) and were attached or "farmed out" on an as-needed basis. Due to the extremely short duration of the 787th's attachment to the 86th Division, the division's veterans association will probably be of little, if any, help (it's worth a try, though). The most recent address I have is:
86th Infantry Division Association 5328 East Calle Redondo Phoenix, Arizona 85018-4438
You may as well try the 16th Armored Division Association while your at it:
16th Armored Division Association Post Office Box 222 Lake Alfred, Florida 33850-0222
The present-day 16th Armor was constituted and organized as 16th Cavalry in July 1916. The regiment underwent a series of changes through the years until it was finally reorganized and redesignated 16th Armor, a parent regiment under the Combat Arms Regimental System (CARS). There is no historical connection between the 787th and 16th Armor.
16th Armored Regiment was constituted 14 January 1943 as an element of 16th Armored Division. Activated 15 July 1943, the regiment was broken up two months later. 1st and 2d Battalions were consolidated, reorganized and redesignated 16th Tank Battalion and remained assigned to 16th Armored Division.
3d Battalion was reorganized and redesignated 787th Tank Battalion and relieved from assignment to 16th Armored Division. The battalion arrived in France less than two months prior to the termination of hostilities, participating in the final stages of the Central Europe Campaign.
The battalion was inactivated at Fort Jackson, South Carolina in February 1946. Redesignated 810th Tank Battalion and allotted to the Army Reseve, the battalion was activated in the Reserve 11 January 1947 with Headquarters at Casper, Wyoming. Reorganized and redesignated 810th Heavy Tank Battalion 22 March 1949. Inactivated 4 December 1950 at Casper. Redesignated 787th Tank Battalion and allotted to the Regular Army 30 July 1951. Disbanded in 1952.
The only way to determine the battalion's location and activities on any given date(s) would be to access operational records. This, of course, would involve a fee.