Yeah, I forgot to add that shipping to EU…
15 euro glass in a flashlight for 1 euro 
I mean something super cheap. Maybe something that you can find somewhere for free.
Must be plastic.
In many sizes, or easy to cut/sand.
Those indestructible clear clamshell packages that toys and such come in, tend to have enough flat surfaces to cut various circles to suit.
Thing is, when trying to open them, they’ll cut like an M-lash. Be sure to know your blood-type.
I bought Inova X5 for 18$ shipped to EU...
why bother?
its a cheap lossy thing.
when i do the high cri and 18650 hack to those you cannot easy use the lens anyway.
i still have a bunch.
postage to get them to you exceeds value of a handful of those lights.
leave it out and enjoy a extra lumen or 2 
In the US, home improvement and hardware stores usually have a small selection of plastics, often including either acrylic or polycarbonate, which a person could cut to size for purposes like this.
In general though, these lights are so cheap few people bother trying to fix them if something breaks.
If you do want to replace a lens for a light that is in some way unique from the rest and you want to keep, the cheapest way to do it is probably going to be to buy another flashlight of the same basic type to swap the lens from.
Go to optician who make prescription glasses in your town. Ask him to make a glass for your flashlight. Plain, coated, whatever.
Mike