Usa-Places.com has listed approx 4 Plumbing supply stores in Upper Marlboro. Some of the Top rated Plumbing supply stores in Upper Marlboro are- Thos. Somerville Co. Trade Zone Branch & Headquarters, Thos. Somerville Co. Trade Zone Branch & Headquarters, Thos. Somerville Co. Trade Zone Branch & Thos. Somerville Co. Trade Zone Branch.