Measure the fitting first
For an opaline tulip shade, begin with the opening, the depth of the fitting edge and the shape of the mount. Add the maximum outside diameter and overall height. A similar silhouette does not guarantee a fit. Photograph the existing holder with a clear scale and explain which measurements have been taken.
Compare glass, colour and edges
Ask for photographs in daylight and inspect the rim where the holder grips the glass. Small chips or cracks in this area matter to the mounting decision. Do not force glass into an unsuitable holder. If several shades will share a fitting, compare their colour, translucency and dimensions individually.
Keep the complete light in view
The former address mentioned Mazda and Art Deco styling; those words alone do not authenticate a replacement. Send the researcher a photograph of the whole light as well as its holder. Have a competent professional confirm compatibility and installation. An approximate visual match may be acceptable, but it should be described as a replacement rather than presented as an original component.
For the next part of your selection, read Antique chandeliers: measure before you buy.