I cannot agree more with Rich D. Having someone make them for you is the cheapest and best way to acquire half-sleeves. However, if you have to have them made, it IS worth the expense. They will save your better work shirts from unwanted stains and/or wear. I speak from experience. There is a reason why the D&S issues enginemen four(4) work shirts per season. My half-sleeves were made for me. A good material is a 100% cotton interlock fabric(black is the preferred color). This can be obtained at any fabric store for around $4 a yard (yes, guys, fabric is sold by the yard not by the foot). You should be able to get three(3) pair of half-sleeves out of a yard of fabric. Another good material is a very lightweight denim. It is warmer but I have found that NO oil or grease can get through that stuff!
Any engineman who does a worthy inspection/oiling should have a pair.
My $.02,
Rob