Historical-market-cap-for-listed-and-delisted-stocks

Market cap is a calculated value.

CODE:
double mktCapMillions = sharesOutstanding * averagePrice / 1000000.0;

So the question remains, which fundamental data provider do you use for the Shares Outstanding?

Personally, since I'm paying for an IQFeed subscription, I use IQFeed for getting the Shares Outstanding. It's not really a "real" fundamental data provider, but the basic subscription comes with a few fundamentals.

YCharts has Shares Outstanding, but the free version is really slow; I often disable it. The paid version is much faster. The paid versions of all the fundamental providers will be much faster. Are you in a hurry?

For a paid subscription, EODHD may be a good choice. If you don't want price data, you can get their fundamental-only data feed for around $20/mo. Well, that's apparently an old price.