Hi @happyday. The important parameters are that this is a 50mm focal length lens. The diameter is 1/2" or 12.7mm. This is not important for the optical performance, but just to make it fit. It is an achromatic doublet lens, this means that it is corrected for colour aberrations, so colour images will be clearer. It is also anti-reflection coated for visible light. Lenses with those parameter should be quite widely available. For example Edmund Optics have Magnesium Fluoride Coated Achromatic Lenses, they are a little more expensive and don’t quite match - either 1/2" diameter and 2" (50.8mm) focal length, or 50mm focal length and 12.5mm diameter. Either should I think work.
You may not actually need the high resolution optics with the tube lens. The low cost optics with the Raspberry Pi camera lens is remarkably good ( Comparison of a pi-camera and a 20x optical lens - Contributions - OpenFlexure Forum. Although looking at your other thread you have previously used a 40x objective.