Is Seungyeon an Inner Circle?
Since he trained under YG Entertainment and boy group Winner is under the same label, it is possible he could be an Inner Circle. The T-shirt is pretty affordable, you can buy it here but they only ship to Korea.
The bucket hat is from Stussy called Big Cities Bucket Hat in black. For some reason, the one that Seungyeon is wearing in the dance practice video has the tag left on XD There are several places to get it, for instance here and here.
Image Credits: Yuehua Entertainment, nastypalm, mltd