im not sure if theres anything else you can do, there doesn't seem to be another way to find em so just keep shaking trees D: hopefully one will show up for you really soon!!!
Make sure you're shaking the right trees during the right times. They only spawn in normal and fruit trees, never cedar trees.
They should be available all day in June, but if you've time traveled here's the schedule for when they show up.
Mar-Jun All Day
Jul-Aug 7P-8A
Sep All Day
Oct 11P-4P
I read somewhere that they will more often spawn in open areas (but that open area around town tree doesnt count), and often near the retail store too. wether that is true or not, idk, but it just happens that I did find my first one near the retail store, in an open area, lol.