The Tamiya product is not usually referred to as an optivisor. I find it listed as the Tamiya Visor.
I mention that as there is a product that has been around for quite some time that is actually called the Optivisor and is a product of Donigan Optical.
https://doneganoptical.com/product/optivisor/ They also have several other products.
I have had one for at least 20 years and couldn't do any modeling without one. Started with the low magnification lens that the standard product product came with and have replaced that with higher magnifications over the years as my eyesight has changed.
You will find similar copies from other Asian sources but I couldn't comment on their quality as I have only ever had an original Optivisor.
A quick search of Amazon show them available there. eBay sellers are another source and I have seen them in some hobby stores. I forget where I got mine.
If you wish to search for other similar products you could try searching for "headband magnifier".
I have seen the type worn by my dentist that look like little binoculars and have often wondered if they would be better, worse, or indifferent for model building but I image as something sold in the medical / dental industry that they would be expensive.
Another option that others have mentioned have been to use high magnification reading glasses that can be found online or at your local pharmacy. 5+ or 6+ diopter reading glasses might just suit. I tried some low diopter reading glasses years ago before I got the optivisor and they work OK for me.
cheers, Graham