I've read that you can find a Ring of Electrical Immunity in Secret of the Silver Blades. (Specifically, Gamebanshee's walkthrough for Pools of Darkness references it under Gothmenes's Palace: https://www.gamebanshee.com/poolsofdarkness/walkthrough/gothmenespalace.php) However, I can find no reference to such an item in any cluebook or walkthrough for Secret of the Silver Blades that I can find. Is it a random item? Does it exist at all? I would like to know for whenever I play Pools of Darkness next.

There is definitely no such ring in Secret of the Silver Blades. In fact, Secret probably has the plainest magic loot of the entire series, despite being by far the most Monty Haul of all of the Gold Box games in terms of loot quantity (this was heavily due to the fact that everything up through Gateway was still being made available on Commodore 64, and as additional spell levels piled in, there became less and less space available for any but the most basic of magic item functions. Death Knights of Krynn is similarly basic in this regard).

It exists in Pools of Darkness (but is bugged on PC, so is useless, which MASSIVELY increases the difficulty of the endgame as a result to the point of almost requiring degenerate gameplay to beat it; it does work on Amiga) and in UA (I forgot if it worked there either or not).

Treasures definitely has support for the item (you can actually copy entire characters over from Pools of Darkness) but I don't remember ever actually seeing one appear in the game. I presume the same is the case for Dark Queen of Krynn's engine (since UA was mostly copied from it) but the actual item does not appear there, either (DQoK in general is insanely spare on magical loot especially for such a high level adventure).

I'm not sure if it's possible or not to get one as a random drop in Pool of Radiance (which actually allows quite the gamut of AD&D items to appear, but only via "random magic item" loot sources, so most people never even scratch more than the surface of what's potentially obtainable there).

I heard there might be some way to get it to work in the DOS version, but it involves something like equipping it during battle, which is probably not something most people would think to do.

The fact that you can get it to work despite a bad bug still means a lot to someone like me who could never win the main game final fight in PoD. This isn't a perfect solution because you are still unprotected at the beginning of the fight, but it sound like it can still make a big difference.