Historically, utility poles have been sold by local or regional distributors. In some timber rich areas, such as the Pacific Northwest, locally sourcing poles is easy, while in timber poor areas like LA, they are forced to purchase poles from places that grow trees tall enough. McFarland-Cascade is a modern producer of treated utility poles, and sources their timber from the Pacific Northwest.

Here are some 1912 advertisements for pole companies. Note the locations, nearly all are from timber rich areas, or are jobbers, buying timber, and preparing it as poles, then marketing them nationwide. So, LA gets their poles from whatever source can provide them with tall enough poles, as opposed to sourcing them from local, native timber. Typically poles are treated cedar or fir, although for a period of time Seattle used untreated cedar poles (poles from the 1940's are still in service), and there is a small untreated cedar pole in front of my home from 1952 that once carried open wire telephone circuits, which now carries the telephone cable.

EDIT: Scroll down another couple of pages to see yet more historical advertising for utility poles