Of course I can't be sure without seeing the entire headset assembled, but from what I see here, this could very well be the right retainer. Retainers are sold by product number or headset brand/model rather than size, and it's been many decades since I bought one, but I suspect this might be a #22 .

The alternative is to measure the diameter of the balls, and pack the headset with loose balls. No great precision is needed because it's probably 5/32" or 3/16". There's also 1/8" but it's not common in headsets. Balls don't come in sized between these, so if you can measure to within 1/32" you're in.