Has anyone been able to find an infant helmet in a size around 42 cm or so? All I can find are toddler helmets, and even Google and Amazon have failed me.

The pedi has ok'd us to put Nora in the jogging stroller and/or the bike trailer as long as we put a helmet on her, and I'm ready to get running again!
Re: XP: Infant helmet
I've read articles going several directions.
Oliver is 5 1/2 months and we bought a giro me 2 helmet. It's super light weight and we put it on him in the Croozer when biking and jogging.
He is totally fine with it.
Caveat - he is STRONG. Everyone says this and he's a BIG boy..big everything. Our ped said it all depends on their developmental process and size. Oliver is bigger than a few 12 months old that I've seen. The giro me 2 has an adjustment head strap.