There is a wealth of material on the 18th while it was in Hood's Texas Brigade, much of it being accounts by Texans, but giving good details on the 18 GA.There is a real lack of archioval materuial directly from the 18th, at least that that I have located.

On the battle of Gaines Mill, in which the 18th played such a pivotal role, here are a number of sources, which should be available to you in Virginia:

Richmond Daily Whig, 1 Juli 62, p. 3, c. 2 (Law's Brigade account)
Richmond Daily Whig, no date, Jul62 (Texas account)
Richmond Daily Enquirer, 4 Jul 62, p. 2, c. 2, 3 (4 TEX account)
Robert Campbell, Lone Star Confederate: A Gallant and Good Soldier of the 5th TexaS Infantry, George Skoch, et al, eds. Colelge Station: Texas A & M Univ. Press, 2003, pp. 35-46,(excellent and detailed reminiscence by Campbell)
Richmond Daily Whig, 10 Jul 62, p. 2, c. 4, 5 (a detailed account by a member of the 18 GA)
Georgia Division, United Daughters of the Coinfederacy, eds., Confederate Rwemioniscences and Letters, 1861-1865, Vol. XVI. Atlanta: Privately Published, 2001, at pp. 234-236. (18 GA account) (I am confident that State Library of Virginia holds this, or perhaps you can get it by interlibrary loan)
Billy Epps, Tithes of Blood: A Confederate Soldier's Story. Murfreesboro: Southern Heritage Press, 1997, at pp. 56-63 (The 18 GA account has also been published elsewhere)
Nicholas A. Davis, Chaplain Davis and Hood's Texsas Brigade ..., originally published 1863, reprint with notes San Antonio 1962, Gaines Mill portion pp. 82-90 This should be available at State Library of Virginia and perhaps in larger libraries; useful)
Richmond Daily Whig, 18 Jul 62, p. 2, c. 2 (4 TEX account)

The microfilm for the Richmobnd papers is widely available, and Unioversity of Richmond has put the Daily Enquirer on-line. There is a wealth of material on Gaines Mill, because members of Pickett's division tried to claim that they, not Hood, had been the troops to penetrate the Feddral front. So the various units in Hood's and Law's brigades naturally responded. I can provide similar information on 2nd Manassas and Sharpsburg, and perhaps a few accounts of the matters in between those fights. This ought to get you started.

The casualty lists for the 18 GA are in the Georgia papers, I think. The Atlanta papers usually published those.