Relevant part (credit to [deleted] and u/chiagod):
Assuming D-T fusion, a single fusion event releases a 14.1MeV neutron and a 3.5MeV helium nucleus. Assuming you can absorb all this energy and you've got an efficient heat engine setup at around 50%, you'll get about 1.5x10^-12 J per fusion, so for a 1GW output you'll need 6.67x10^20 fusions per second. Say you have 1TWe (electric output) worth of fusion reactors worldwide (about half of current electricity generation), then you're producing 1000 times as much helium, or 6.67x10^23 atoms per second. About a mole each second, or 4 grams. This works out to 126 tons of helium a year, or about 1000m^3 per year of liquid helium. The US strategic helium reserve had a peak volume of about a billion m^3 . World consumption of helium is measured in tens of millions of m^3 per year so you'd be short by several orders of magnitude in the best case.
Helium is found adjacent to natural gas deposits which have formed in rock with a high uranium content. Over time, the uranium decays, and helium atoms are part of the result of that decay. Almost all of the world's helium comes from an area around San Antonio, Texas.