Where you can get your 45+ deck is at the Bazaar (55 max. cards) or from boss drops (55 max. cards & 60 max. cards).
Decks that hold 55 max. cards, 7 max. of your school and 4 max. of others:
Myth: Karuvian Deck of Eternity [drops: Karuvian Scargiver in Plaza of Conquests]
Life: Deck of Nature's Bounty [drops: Firevein in Plaza of Conquests]
Storm: Sea Lord's Deck of Thunder [drops: Sea Lord in Plaza of Conquests]
Ice: Deck of the Ice Wyrm [drops: Sandor Spearcaller in Plaza of Conquests]
Balance: Deck of Perfect Harmony [drops: Marva Flamewing in Plaza of Conquests; Valerik Brightsword in the Crucible]
Fire: Deck of the Devouring Flame [drops: Tamris in Plaza of Conquests; Iona Pyrelance in the Crucible]
Death: Deck of Last Departure [drops: General Firetusk in the Grand Chasm Past]
Decks that hold 60 max. cards, 7 max. of your school and 4 max. of others:
Myth: Deck of the Silver Sentinel [drops: Silver Sentinel in the Labyrinth]
Life: Deck of the Gallium Paladin [drops: Gallium Paladin in the Labyrinth]
Storm: Shadowcrown's Hurricane Deck [drops: Devora Shadowcrown in the Labyrinth]
Ice: Blackrock's Obsidian Deck [drops: Boris Blackrock in the Crucible]
Balance: Steelwielder's Metalbound Deck [drops: Zora Steelwielder in the Labyrinth]
Fire: Bristleback's Singed Deck [drops: Andor Bristleback in the Labyrinth]
Death: Grimcaster's Doomfaring Deck [drops: Orin Grimcaster in the Crucible; Bael Emberlight in The Crystal Grove]
*Notes: - 60 max. cards decks are No Auction. - Don't count on my spelling lol. - 55 max. cards decks can be dropped from the first boss of Briskbreeze Tower, in Colossus Boulevard.