I believe there are 4 (Maybe 5 if he was in lucky stars, I can't remember) main events that I've seen host Shisui:

Lucky Board

Great Plates

Myobuku Wheel

Treasure of the Sage

Personally, I think if you're not in a hurry to grab him, try to get as many free frags from Great Plates as you can before you buy him. Each frag you obtain will save you ~135 coupons. After you've collected enough frags, purchase him from Lucky Board so you get some free power items. However, in the case that he miraculously appears in Treasure of the Sage again for a max discount of 30%, you should go for the discounted as it is much cheaper.