"Spell Focusing" is a new property that is only available on the sash from the war/invasion event, from killing Ophidian/Meer that are part of each town invasion wave.
"Casting Focus" is the property released with Stygian Abyss that allows you a chance at not being interrupted when you otherwise would have been. It is available on the following:
5% from GM Inscription.
2% from Crown Of Arcane Temperament. (A Stealable circlet located near the Cavern of the Discarded - you need around 110 Stealing for a reasonable chance.)
3% from the Protector Of The Battle Mage tunic. (A leather tunic drop from the Primeval Lich.)
2% from Warded Demon Bone Bracers. (Medable Bone Arms, one of the possible rewards from the first part of the Bane/Ophidian war - now over - which took place in the desert North of Papua.)
2% from Leviathan Hide Bracers. (Require High Seas booster pack to wear, are marginally better than the Warded Demon Bone Bracers, I don't know what type of sleeves they are but presumably medable. They come from Corgul or Scalis.)
4% from Enchanted Kelp Woven Legs. (Leather legs from the improved Message In A Bottle chests, the drop rate is overly high so are usually extremely cheap.)
Casting Focus is also available on the gargoyle only artifacts; Mantle Of The Fallen (3%), Summoner's Kilt (2%) and Obsidian Earrings (4%).