I've never bought oyster shells. I'm not opposed to doing so, I've just never seen the need.
Yes, I feed a Layer formula. That accounts for much of it. Yes, I feed back eggshells which accounts for the rest, I suppose. Also, good quality, calcium can be taken up in a powder or tiny pellet form as well. Since we garden commercially, we have bags and bags of calcitic lime for the fields. If I ever find the shells getting a bit thin, which is a rare event, that's always available. Be careful, though, as most lime for gardening is the wrong kind, both for the garden and for the birds. Way too much magnesium.
Remember too that other factors go into a hen's processing of calcium and building strong shells. There has to be Vitamin D and a balance of phosphorus as well. The only time I've ever noticed the hens lacking Vitamin D is in winter, usually late February to mid March when we can get days and days of gloomy skies. That's really tough on them.