Rubber, maybe silicone? I bought 10 pairs from Northern Safety that took FOREVER to ship, but they're still $1.25 a pair or so. https://www.forestry-suppliers.com/p/94449/40770/tasco-tri-grip-reusable-earplugs
They do start out a little stiff, even with the cord out, but they quickly soften up after wearing them on a ride. I can tell the difference in a new pair but it's not an issue at all. I get more discomfort from the bottom of my ear being pressed against my head from my cheek pad.
Edit: be sure you do wash them occasionally, or you'll end up being unable to grab and pull them out of your ear unassisted. Not that I figured that out on my own. A paper napkin helps add friction while you grab if you're sweaty though, or so I've heard.