I sometimes use a D3+calcium supplement for breeding females but I'm not really convinced that it helps with egg shell formation. I've had dodgy shells from females that have been supplemented for months before laying, and great eggs from unsupplemented females. I think I give it for my benefit rather than theirs!
Calcium can only be metabolised if the snake has enough vitamin D3 in their system. Without enough D3, the calcium just goes in one end and out the other without doing any good. For that reason, I always choose a supplement that's D3+calcium combined and I try to start it round about mating time rather than waiting until she's gravid.
Here in the UK I normally buy "Zolcal-D" but not sure what the US equivalent would be.