Tickets for Shinahachi are allocated to certain sellers such as Yoshimoto tomonokai and Idoling!!! Fan-sama club. The allocated tickets almost always sells out because club members buy from these sellers. These fan club members get to buy the tickets a week earlier I think. You can also buy it normally from tickets sellers but these sold out fast too because touts are buying them from here.
Even though some place may sell out, you can always buy the 当日券 which sells at the day of the live but you need to get there early and the seats are not as good as the fanclub tickets.
The 9th live and Countdown Party 当日券 didn't even sold out. Hatix managed to buy a Countdown Party ticket just before it started and I've heard Kadosawa P himself was helping to sell the leftover tickets
As Idoling!!! got more popular tickets are gonna be much harder to get and are going to be more expensive on auction
I used to be able to buy the ticket normally but lately I have to buy it on auction because the demands are getting bigger.