is there anything commonly sold that converts to this kinda like sodium bicarbonate can be converted to sodium carbonate?

Seems to be all over that auction site.

OTC maybe at reptile suppliers?

Calcium carbonate is everywhere, but if you're thinking of using it as a base: it's not like sodium carbonate at all. Theoretically you could convert it to calcium oxide by heating it, and then add water to make calcium hydroxide. Not very practical, it needs to be at around 800 C, orange-red glowing.

You can heat sodium bicarbonate in an oven for 30 mn to get sodium carbonate. But it might be already sold but under another common name. In france it is commonly sold in all supermarkets as a detergent product for cleaning everything in the house.

I'm just gonna use it for sublingual stuff thanks guys I'll hit up the vitamin stores and maybe wineries.

I've seen calcium carbonate in home improvement stores being sold as simply "Lime". It was in the garden section, though I'm not sure what it's used for. I was looking for calcium hydroxide, so I didn't pay it much attention.

I was in walmart today and it look like most calcium pills were made of calcium carbonate.