If you follow these instructions to add your location It will help others give you better advice. (be as general—UK; or as specific—town, etc as you are comfortable with)
A vet could get you something, yes.
It doesn’t have to be calcium Gluconate— calcium carbonate or calcium citrate are fine too.
The same product you can get at the drugstore for yourself, you can use that.
Chewable antacid tablets contain Calcium. (Known as Tums in the US)
You can also use a human otc calcium supplement. Even Tums (yes, the antacid) can be used. The trouble with dosing in the waterer, is you can’t be certain how much she’s getting.
Give her 2 tums to start, then 1 each day until you see a hard-shelled egg or at least 3 days.
Other calcium...