Take a look at these dual port, fixed chemical injectors by General Pump. 

Operating Pressure - 5500 PSI

Maximum Temperature - 190 degrees F 

Port Sizes - Inlet: 3/8” NPT-M Outlet: 3/8” NPT-M

Available in Brass and Stainless Steel 

3-5 = 2.1mm orifice

5-8 = 2.3mm orifice