The easiest method(but expensive) would be to purchase a wand of Bow Spirit(30,000gp). If you are playing in a campaign wherein you can get items created for you(using the Core Rules Magic Item Creation section), you could get an item that could cast the spell twice a day for only a bit more than the wand: Command Word/2 Use/day item formula Spell Level=4 Caster Level=13 Base Cost=1,800 Twice/Day=2.5 Total Cost=4131,8000/2.5=37,440gp

The easiest method  (but an expensive one) would be to purchase a wand of Bow Spirit, which costs 30,000gp. If you are playing in a campaign wherein you can get items created for you  (using the Core Rules Magic Item Creation section), you could get an item that could cast the spell twice a day for only a bit more than the wand:

Command Word/2 Use/day item formula
Spell Level: 4
Caster Level: 13
Base Cost: 1,800
Twice/Day: 2.5
Total Cost: 4 × 13 × 1,8000/2.5 = 37,440gp