Friendly neighbours to the north[1]. We don't like tooting our horn enough, but America gets the bulk of its energy raw materials from us.[2]

[1]https://www.eia.gov/todayinenergy/detail.php?id=2150

[2]https://www.eia.gov/tools/faqs/faq.php?id=727&t=6

Austrilia is #1 by reserves, Kazakhstan is #2 and Canada is #3.

US as of 2012 buys about 25% of it from Canada and the rest from a variety of places.

https://en.wikipedia.org/wiki/Nuclear_power_in_the_United_St...

https://en.wikipedia.org/wiki/List_of_countries_by_uranium_r...

"In 2011 the United States mined 9% of the uranium consumed by its nuclear power plants.[7] The remainder was imported, principally from Russia and Kazakhstan (38%), Canada, and Australia.[8][9][10] Although uranium production has declined to low levels, the United States has the fourth-largest uranium resource in the world, behind Australia, Canada, and Kazakhstan.[9]"

https://en.wikipedia.org/wiki/Uranium_mining_in_the_United_S...

Major imports into the US come from Canada, Kazakhstan, Australia, and Russia. Namibia used to be a bigger import source country, but it's apparently dropped off in recent years.

Source: https://www.eia.gov/uranium/marketing/pdf/umartable3figure5....

I don't think it's particularly hard to get since quite a few countries have viable reserves.

It's the enrichment which is the tricky part.

Australia has plenty of Uranium.