Sheldon's Undershirts
Sheldon often, if not always, wears a long or 3/4 sleeve shirt under his main t shirt. These shirts are usually in solid colors (but sometimes striped) and we can only assume that they’re just plain long sleeved shirts. Below are some links to places you can buy similar shirts (as identifying the exact brand would be pretty impossible without the help of the costume designer).
Check it out →