Concert-oriented earplugs work fairly well for this, and they're pretty easy to find. They aren't safety-oriented so they don't cut by the ~30db of "normal" plugs, more often around 15db.

Generally they (claim to) try to sound more neutral so it's "just" a reduction in volume without much tone bias - personally I can't tell and haven't bothered to check, the small volume cut is all I really care about and any tone bias would be small enough that it's tough to notice anyway.

You can of course go much more specialized for many specific goals, but start cheap and simple. $20 or less is easy to find, though you might want to hunt around for comfort purposes (e.g. personally I only find completely soft ones comfortable for long periods, and anything with a long hard stem that pokes out gets bumped and seems like a safety hazard to me).