I also really like Toski, Bearer of Secrets , mainly because I do not own a copy of Ohran Frostfang
Like most Magic: The Gathering cards, the value of Keruga, the Macrosage depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Ikoria: Lair of Behemoths Promos)
You can pick up individual cards from your friendly local game store and from secondary markets like TCGplayer online, or you can pick up booster boxes from Amazon if you fancy cracking packs
First and foremost, blue has a host of tutors for instants and sorceries
You can only use each effect of "Starry Knight Rayel" once per turn
This is a solid enough effect at a hybrid cost that stapling it onto a dual gives it a ton of utility