page monitor extension for Chrome, I follow various twitter feeds and have push notifications set up, same with instagram and facebook (although I haven't really figured out the best way to actually get alerted to those, I still randomly check them throughout the day), and there are a few different websites that people use during the holidays to find like SNES Mini consoles and stuff like that, but they're generally not great for Pops. I also bookmark placeholder pages and randomly check throughout the day - I check more often if I really want something.

Also if you see that a site has other exclusives restocking it never hurts to check on other site exclusives, too, because they usually do that stuff in groups.

Walmart in particular has gotten very hard to monitor - I just lucked out with this one because I was looking for Zamasu Glow and just randomly checked Thanos.

tl;dr - you kind of have to be obssesive complusive to have luck with restocks