According to The Online Tank Museum and US Marine Corps 1941–45 by Gordon L. Rottman, both cited in Wikipedia's Boys anti-tank rifle, the marines got them from Canada.
This American Rifleman article gives more details,
At the time of the attack on Pearl Harbor, the American military was
woefully short of many types of modern arms, and there was no infantry
anti-tank weapon in service. The M2 Browning .50-cal. machine gun
filled that niche throughout the 1920s and 1930s, but the M2 with its
tripod weighed about 128 lbs., which precluded it from being carried
by one man for any distance.
In the bleak early days of America’s war in the Pacific, the U.S.
Marine Corps formed Raider units, loosely based on the British
Commandos. Raiders were trained in “hit-and-run” tactics that required
easily portable arms. The Raiders needed an anti-tank arm and quickly
settled on the Boys. To this end, the Marines standardized the Boys in
The Boys, although heavy, was lighter than the M2 Browning .50-cal. An article by Major Jon. T. Hoffman, USMCR, From Makin to Bougainville: Marine Raiders in the Pacific War has this.
Perhaps the oddest weapon carried by the raiders was the Boys antitank
rifle, a 35-pound behemoth firing a .55-caliber round. Edson adopted
these Canadian weapons to provide his men with a light but serviceable
capability against enemy armor. The rifle eventually saw use with
other raider battalions. The heavy round was accurate at more than
1,000 yards, and the 2d Raiders used a Boys on Makin to destroy two