In the Global Trade, these storage items, which are essential for the expansion of the storage, appear very frequently on Tuesdays throughout the day.
The reason was explained in another trick (When to buy in Global Trade). But also in a small city, not seeing so much variety of products, they appear very easily, so much so that you can get an average of 30 SI items as soon as you spend some time on it these days (*).
This means that you can expand small cities every week without effort.
(*) Note: As there is a change in the basic maximum capacity of the storage in September 2022, finding storage items from this date until a few months later, will be much more difficult, and this trick will not be effective until a reasonable period of time passes, in which the cities with maximum capacity reach the limit of the storage again.