Someplace in China however I’m not sure you’d be able to deal with them due to Chinese human rights violations. If that’s not a problem try DX. You’re looking for o-rings and I don’t know what you call the insulator plates or if they carry that.
There is MacMaster Carr in the U.S. for many parts but not specifically for flashlights. However, you might not like dealing with an American company due to all the people killed in the name of U.S. supremacy.
You might try a Swiss company or maybe you’d feel more comfortable with a Croatian company. No names come off the top of my head at the moment however.
The easiest thing (I’ve found) regarding o-rings and DX is just to order all of them. They are cheap. Or at least any that they carry that you could possibly use.
If you need one in a hurry you might be able to find some in a local hardware store. I don’t know what’s available in Croatia but there is a chain of hardware stores in the U.S. called ACE Hardware and they have a flat file type of cabinet in most stores with a pretty good selection of both imperial and metric o-rings.
The interesting thing is that ACE Hardware stores are the smaller (local neighborhood) stores. I can’t find o-rings in Home Depot or Lowes (the largest stores).
You buy such isolators? Why don't you build them by yourself. I cut them out of a thicker piece of paper. It isn't important for me, that it has no glue. If I need glue, I use 3M Spray Mount (this is a special glue for things which should be removed).