It's in Player's Option: Spells and Magic.
Here's the text of the ability Caelic used:
At 7th level, the artificer may create a temporary magical item. Any magical item in the DMG not specifically restricted to nonwizards is allowed, but the item will function only for the artificer. This is a special ability unrelated to the enchant an item spell. First the artificer must successfully research the item creation process, taking one week per 500xp value of the item and spending at least 100gp per week. This time is halved if the artificer has a sample of the item to copy or if he succeeds in a contact other plane, legend lore or other research spell. The artificer must passs a learn spells check to succeed and may never know the processes for more magical items than his maximum number of spells per level. Actually building and enchanting the item requires half the research time and 2d6x100 gp, plus the cost of the item iself. Fine materials must be used, but rare and exotic materials and pocesses aren't necessary for temporary items (see Chapter 7). After completing the work, the artificer must pass another learn spells check to successfully enchant the temporary item. A temporary item lasts 1d6 days, plus one day per level of the artificer. Once the enchantment fades, the item can be re-enchanted with one uninterrupted week of work, the expenditure of 2d6x100 gp, and another elarn spells check. If the temporary item normally possesses charges, the artificer automatically places one charge per level into the item when creating it.
The Arrow of Slaying has a 250 XP cost, so it would take 1.75 days and 25 gp to research, and 0.875 days and ~700 gp to create.
Interestingly, you could create a temporary Manual of (insert stat), since they are not specifically restricted to non-wizards, but normally outside the realm of player-creatable stuff