Whenever I go into a bookstore, I usually see SJ from a few different months in the manga section on the ends of the shelves. Usually, they're months around the time you go in.
If you went in to check, there might be something.
Otherwise, maybe there's some selling on Ebay, or you can get a hold of VIZ customer service, and see if you can order from them.
https://magazineimpulse.com/shonenjump : That's the January issue.