we already have eggshells readily available from our chickens
i use this (bigger in size though) to crush the eggshells (crush first dry later or dry first then crush.. it's up to you)
but since the layer feed i'm giving my hens provide 4% of calcium.. i didn't have to do this a lot.. plus...
i was in the same situation as you 4+ week ago
https://www.backyardchickens.com/t/1121551/our-very-first-egg
what i found out was
it's ok to continue to feed your hens starter feed
their eggs will not get to full size until 4-5 weeks after the 1st egg
you can feed all hens starter feed...