If you’re not taking it to the LBS to do, then give Wych Bearings a shout for sure. They’re based just up the road from me, and I use them as my local bearing supplier too. In all the years I’ve been playing about with bikes, they’re the only bearing supplier out there that seems to even remotely understand the requirements involved from bearings within bicycles, rather than in machinery. They keep good stock of all the popular sizes too.
Be careful though as headset bearings are a nightmare. If you can’t read the bearing code off the outside face of the bearing (common when they’ve corroded inside a headset), then measure the outside diameter, inside diameter, depth and the angle faces and match them up. There’s soooooooo many different types of bearing for headsets, it really isn’t just a case of 1 1/8th or 1.5″ as you expect it might be, and they’re not cheap either!
Mine did the same, I took it in to the local place that looks after the bike and had it swapped for a hope one, much better supply of spares when they need doing in the future
There’s good merit in this. Not because of spares availability (Hopes bearings are normally proprietary, so have to be bought from Hope, so often are harder to get hold of), but because Hope Headset bearings normally last a lot longer than cheap FSA or the like fitted to most OEM bikes.