You can actually get anal plugs that have ball bearings in them that bounce around as you move, so you get vibration without electric parts.
But ultimately, it's just something you've gotta try a few different things until you find something that works for your body. There's a few hundred silicone prostate massagers out there.
The way I searched was using AliExpress. They're the exact same thing as the $10-20 ones on Amazon for $2-5, if you can wait a few weeks.
I'd recommend getting a set to start with until you find what insertable length works best for you. Unfortunately, whomever designs the toys seem to think that longer massagers need to be thicker too, so you might need to work up to one.
You'd almost have to intentionally search for toys these days that aren't silicone, abs, glass, or metal, though.