do you mean 300w equivalent, or actually 300watts of power drain?
Im using 3x20w (100w equiv) between 2 plants and they are loving it.
Just be aware that using low wattage cfl's you will get a better yield if you keep your plant small ie you should switch to flowering as soon as you get your 5th set of real leaves.
a plant matured longer and bigger in size under weak cfls will produce inferior buds when compared to a small plant in the same light due to bigger plants using more energy overhead just to exist.
The answer to your question "is a 300 watt bulb big enough to grow 1 plant", is yes it will grow fine. Though more information on how you plan to grow would allow you to receive a much more informative answer.
The answer to "where to find them" is sitting right in front of you. Just use that wonderful device called a computer and you can search massive amounts of places for what ever you are looking for.
A couple of thoughts on using a single 300 watt cfl:
1. 300 watt bulbs are harder to find (think about a bulb dieing in the middle of a grow)
2. twelve 23 watt bulbs give the same lumens at a little less consumption. (small grows do very well with SCROG setups)
3. two 150 watt HPS will give you around 70% more lumens.(though more heat to deal with)
You can get the 300watt cfls at home depot they range from $14.97 to $19.57. I was going to use the 300 watt bulbs aswell but decided to go with 8x100 watt equivelnt cfls instead, good luck hope it helped