You can check merchants each time you level for weapons or armour with higher level enchantments, and a couple can carry enchantment formula (though usually not as high as you can get from equipment). Save before approaching merchants (the first time you talk to them after leveling) and you can reload a couple times to see if they offer anything you can use.
Equipment and formula can also show up in chests and as quest rewards.
Other than some of the DKS set pieces or unique equipment, there isn't really anywhere that you can go get a specific enchantment. Even then, when there are pre-determined enchantments, they tend not to be higher than you can find randomly.
From a search of Lancet's
DKS FAQ/Guide, there were a couple weapons with Increased Magical Damage in Broken Valley, a bow with level 1 can be bought in High Hall after you do a quest there, and then there are more once you get to FoV.
