The NERF™ Super Soaker CPS2700 has a liquid volume capacity of 4 liters, with an additional 800 mL capacity in the pressure chamber. It goes on eBay for as low as $60.
Mail-order holy water, by the mL, costs on average $0.63.
Therefore, if my math is correct, it would cost roughly $3084 dollars to procure and completely fill a super soaker with sanctified water.
For the price of a down payment on a used car, you could instead be styling on demons from a comfortable distance in rad Y2K fashion.
Now to figure out the logistics of salt grenades...