As of Feb 1, 2022, they have space for about 100 booster shots per 15 minute slot, every day for the next two weeks, day and evening. But you need the voucher from your municipality to be able to sign up. Vaccine is moderna.
Eligibility is 6 months after your 2nd shot.
Sendai City has sent vouchers on Jan 31 to all Sendai residents who received their 2nd shot by July 2021. They will send vouchers on Feb 14 for people who received 2nd shots by Aug. 14, and they will send vouchers on Feb 28 for people who received their 2nd shot by the end of Aug.
In addition to the Miyagi mass vaccination site above, there is also a group vaccination site at TKP Garden City near Sendai Station (Pfizer):