While there is no specific name of the site you would like to visit, I assume that it may be Jongmyo Shrine considering the Japanese guided tour and ticket purchase time schedule you mentioned.
If it is Jongmyo, please visit the site 10 to 20 minutes earlier and buy the tickets on site. Since the ticket office usually opens earlier than 9:00 am for the visitors, you will be able to enjoy the guided tour on time.