Chamber flag or chamber safety flags. Many new guns come with them. My club requires that they be inserted between shooting sets during competitions.

During a stop at my LGS, I asked if they had any, and the salesman looked around on the floor behind the counter and came up with several, enough types and sizes to cover all of my pistols and one of my wheel guns.

The club supplied flags for those that didn't have them for last week's military rifle shoot. The other thing they'll accept as a flag is a length of florescent string-trimmer plastic "string", inserted through the breech into the barrel, so the range officer could easily see that the action was open and no round was in the chamber. This shows up even better than many of the purpose-made versions, which may be quite small, works very well as long as the slide/bolt is locked back, and is super fast to remove when the load command is given.