Even though you can buy them in Canada I belive there may be an issue to take thermal optics out of the USA under ITAR regulations without an export permit.:
Export of Thermal Imaging Equipment or related accessories (such as manuals) is strictly regulated by the US Department of State in accordance with the guidelines of the International Traffic in Arms Regulations (ITAR). It is a major crime to ship or carry US manufactured thermal imaging devices outside the borders of the United States.
The Arms Export Control Act (22 USC 2778) governs the export of articles, services, and technical data identified on the U.S. Munitions List. All night vision equipment systems, components, goggles, and weapon-sights of Generation 2, 2+, and 3, as well as infrared artifacts, illuminators, and aimers, are currently identified as U.S. Munitions items.
Dan