I'd start at the Tennessee State Library & Archives in Nashville. Find out what units were along the Elk River and in what time line. Procurement records are often difficult to find. Purchasing was legal at almost every level of command, from company to corp and of course state and central government. If Patterson was able to meet the needs of a brigade or higher your chances of finding records is far better, but at lower levels it might prove to be rather difficult. Consider too, if he was able to provide only raw hides or basic tanned products records will be difficult however; if he was able to produce finished product, ie saddles, tack, (often tanners employed wagon makers) etc in quantity a contract would be more easily found. Good Luck!