Kind of up to you. Nothing wrong with buying a couple boxes of factory ammo for barrel break-in. I'd pick ammo that has the brass you'd want to use. If you're wanting high end brass, then you might be better off just loading your own.
The last rifle I did barrel break-in on, I used the bullets and powder I was hoping would work out. I ended up shooting a full box of virgin Nosler brass (50ct), but you don't need that many to break in a barrel. I was also able to baseline my velocity and sight in my scope.
Then, after that I had 50 pieces of once fired brass for accuracy development. and I already had some ground work done since I was using the bullets and powder that I wanted. It just maximizes your time at the range, and the money you spend on ammo.
