Where can you buy 0.25 inch inside diameter silicon-steel tubing?
Check your local hardware stores, if they don't have it they may be able to order it.
Three-quarter inch diameter pipe refers to the outside diameter. Different pipe types of the same outside diameter usually have different inside diameters. ASTM SCH 40 pipe of 3/4 inch diameter has an inside diameter of 0.824 inches. ASTM CL 200 pipe of 3/4 inch diameter has an inside diameter of 0.930 inches because of the thinner wall.
Above 4-1/2 inch is called Casing. Below 4-1/2 inch is called Tubing. Both measured by Internal Diameter. CASING-Is a large diameter pipe that is assembled and inserted into a recently drilled section of a borehole TUBING - Small diameter pipe which is installed in the casing. Oil is produced through tubing because it increases the viscosity of fluid and a well's flow capabilities
1/12th inside diameter plus 1 inch for 'B' wall (most common type), additionally, pipe sizes are normally in 1/4 or 1/2 foot increments, so the closest size would be a 24 inch diameter with 3 inch wall (24/12=2+1=3) having a outside barrel diameter thickness of 30 inches for 'A' wall 1/12th inside diameter, 'C' wall is 1/12th inside diameter plus 1 3/4 inch