For Western Larch, see John's reply. For Alpine Larch, the closest is probably the upper reaches of the Bitterroots. Alpine Larch, common at timberline in the Selkirks, Canadian Purcells, the Canadian Rockies, and the colder eastern portions of the North Cascades, doesn't get much into Idaho except N of the Salmon River drainage on the very highest and wettest areas. Fun fact: Western Larch is the only larch worldwide that is NOT a timberline species.
I'm not, however, a very good authority on the higher Idaho ranges. Has anyone ever seen Alpine Larch in, say, the wetter Sawtooths?
Ski, climb, kayak. Repeat.