Ennyways, a good basic working artillery uniform for you would be the 1884 trousers, a blue shirt, black boots or black shoes and leggings, a tan slouch with an optional red worsted wool cord. No stripes on the shirt, get the 1884 tunic for that.
The Quartermaster shop has this, or you can get brown duck pants and a nice blue military shirt from River Junction. (The Cotton Flannel looks good and is great in hotter weather.) Hat and Leggings from Coon Creek Old West. The Marine Leggings from What Price Glory work well too!