Probably at big university libraries. If you want to get ones to keep and no library is purging copies of years wanted maybe you can buy some from the magazine itself.

Huh. I'm really surprised it would be more popular up here since it's so often California focused (which makes sense given population/territory size differences between there and here). I love it (I just got this year's experimental Mimulus hybrids from their new plant introduction program) and have purchased a few back issues on Ebay so that might be a good resource for you if you can't find a good trade. Good luck! :)

Here is a link that might be useful: Pacific Horticulture on ebay