Dropps are also available at Linens and Things and Bed, Bath & Beyond.
You can also use dish detergent...just a small squirt though, as it bubbles a lot more than laundry detergent. When I was growing up, the lady across the street used dish detergent for everything...clothes, dishes, washing hands, washing hair, showering... And WDW gives you a small bottle of dish detergent in the room.
I also like the idea of putting a washcloth into a Ziploc, and putting the right amount of liquid detergent in there and letting the washcloth absorb it, and then just throwing the washcloth into the machine with the clothes. If you think you'd be doing 3 loads of wash, you could do 3 washcloths.
They do sell small boxes of laundry detergent at the resort gift shops...it's pricey, but it's a once a year thing, so it may be worth it not to have the hassle.
There's also the option of taking the bus to either the Boardwalk or DTD and walking across the street from either one of the to the Hess Express gas station which also has a mini mart where they sell that kind of stuff.