Right angle prism is the most widely used prisms. It utilizes the inherent total internal reflection (TIR) at the hypotenuse to invert the image and achieve a 90° change in direction. This produces a left handed image and depending on the orientation of the prism, the image may be inverted or reverted. Right angle prisms can also be used in combination for image/beam displacement. It is typically coated on the hypotenuse with Al /Ag/Au to enhance reflectivity, while the input and output faces are coated with MgF2 (a common AR coat).3mm/5mm/10mm/12.7mm/20mm/25.4mm/30mm could be quick supply from Yutai Optics.  Yutai's suggestion for your choice: a.Right-Angle Prism with AR Coating on the hypotenuse if you wish to use your prism as a retroreflector and you want to reduce surface reflections over the specified wavelength range of the coating. b.Right-Angle Prism with appropriate AR Coating on the legs if you wish to use your prism as a 90 degree reflector and you want to reduce surface reflections over the specified wavelength range of the coating.