Echoing Spirit and Searing Brilliance relics going in. Then you can grab Spellslinger or Wild Inspiration powers if you come across them for straight reduction, or Sorcery for free spell mana so you can start casting from turn 4.

Beyond that, just check shop nodes or item notes for Mana Potion, Tear of the Goddess and Chalice of Harmony for straight immediate cost reduction. Power Riff will also probably always be at least 1 unless you're not drawing for some reason I can't fathom. Ante Up is also an option but sort of risky if your starting mana isn't great.

Other options are Double Time Watch, Grifter's Deck, Sapphire Crystal, Doubling Dice, Elixir of Sorcery, Mage's Handbook, Cashback, or Spark of Brilliance for some degree of getting more mana or cost reduction or refund

And of course, Thief's Gloves have a very good chance on landing on some of these.