Extra episodes of Golden Douga Gekijou included on the Blu-ray / DVD volumes 1, 7, 8, 9, 10, 11 and 12.
H!Anime is the best site to watch
Golden Kamuy: Golden Douga Gekijou Specials SUB online, or you can even watch
Golden Kamuy: Golden Douga Gekijou Specials DUB in HD quality.
You can also find
Geno Studio anime on H!Anime website.