How to get slugs I don’t see in the shop – Slugterra: Slug it Out! Review

I keep on looking on you tube and I keep seeing night geist,goon doc,etc but I can’t seem to find them in the shop so I’m wondering if someone can help get those slugs and ghouls like tell me a way to get them thank you this game is very fun and I love it please don’t stop working on this game it’s wonderful
Review by great game but how on Slugterra: Slug it Out!.