Other countries to sell us this vital material
In the United States manganese can be found mostly in Maine and Minnesota. However, according to the USGS, the quality of these deposits are are so poor that "they are presently uneconomic to mine." As a result, the United States imports all of the manganese it uses. According to the USGS, the "net import reliance as a percentage of apparent consumption" is 100%. It's been 100% since 1991, the oldest data available from the USGS's website.
That's a long time and shows just how reliant we are on other countries to sell us this vital material. The largest manganese producers are South Africa, Australia, China, and Gabon (a relatively small country on the west coast of Africa, the continent). Those four nations accounted for 70% of the world's production. The countries we buy from, meanwhile, are Gabon (61% of U.S. supply from 2008 to 2011), Australia (21%), South Africa (7%), and Brazil (5%).
Those supply stats are a little bit frightening because just cutting Gabon out of the equation would quickly push the United States into a deep manganese shortage. That said, there's no reason to expect such a problem and other countries could make up for such a shortfall, since there is plenty of manganese in the world (it's the 12th most abundant element). But it wouldn't be easy and other suppliers may not be as willing to sell to us as we hope.
For example, China has to supply it's own steel industry with the vital metal. Australia's mining industry, meanwhile, is focused around serving the Asian market. Increasing shipments to the United States could mean trimming shipments to Asia over the near term. That would probably be undesirable from a customer relationship standpoint. That leaves current U.S. suppliers like South Africa, which is home to roughly 70% of the world's known manganese reserves, and Brazil. While both could step in to help, current supply relationships with other nations might be problematic over the near term if we were looking to offset a huge shortfall.