Do not be fooled into buying an "air cooler" instead of a portable air conditioner. The portable air cooler is tempting, because it is cheaper. But it is not going to lower the temperature of your small room more than one or two degrees. An air conditioning unit that is more than a fan is what you need.
Consider going with the self evaporating style small room portable air conditioner unit. Chances are the extra room you need to keep cool is not going to have a sink. Worrying about emptying the water is just too much of a hassle.
Keep in mind that the higher the BTU number, the greater the portable air conditioner's cooling capacity. Using a portable air conditioner for a small room is an efficient way to save energy.
Look for a portable air conditioner with an EER (energy efficiency rating) of ten or more. Staying cool and saving on your energy expenses, sounds like a win win.
Use helpful online resources to compare portable air conditioners. I have provided a helpful link below under resources to help you find a portable air conditioning unit that fits your needs.