Americium-241 is a man-made radioactive material. It was discovered by scientist working on the Atomic Bomb Manhattan Project during World War II. Americium-241 is a "by product" of the nuclear fuels process and thus the Nuclear Regulatory Commission sets requirements for the production and use of Americium-241. The NRC maintains a list of licensees under 10 CFR Part 32 - Specific Domestic Licenses to Manufacturer or Transfer Certain Items Containing By product Material. Many smoke alarm companies do not manufacturer smoke alarms in the U.S. The smoke detectors may be manufactured abroad and imported or "transferred" to the U.S. Sometimes the components of the smoke detector (plastic, electric circuit board and ion chamber) are manufactured abroad and assembled in the U.S. When ionization smoke alarms are imported into the U.S., the company must meet the NRC license requirements for gas and aerosol detectors at 10 CFR 32.26.
Any smoke alarm that you buy that contains radioactive material should contain a notice on the packaging. For example, a typical package labeling might read:
"This product is designed to detect products of combustion using ionization technology. It contains 0.9 microcurie of Americium 241, a radioactive material"
"Distributed under US NRC License No. _________. Manufactured in compliance with U.S. NRC safety criteria in 10 CFR 32.27"