In a place that has an eruv, can one carry a shofar to shul on Yom Kippur (e.g. during the break between Musaf and Mincha) so that it can be used at the end of neilah? or is there an issue of muktzeh, preparation for the next day (since the shofar blowing is at the end of the holiday), or any other issue?
The Rama 308:4 says that Shofar is Muktzeh as a KLi Shemalochto Lissur:
שופר אסור לטלטלו כ"א לצורך גופו או מקומו (הגהות אשירי סוף פרק במה מדליקין)
It is forbidden to move a Shofar unless for the sake of using it or for using its place. (Ha'gahot Ashiri, end of Perek Bameh Madlikin)
Since it is assur to blow a shofar on Yom Kippur it would be impossible to say that it is being moved for the sake of using it and therefore would be muktzeh.