What's the long term plan? I think your heart is in the right place but the logistics aren't. Is the cat friendly? If so, s/he would probably be better in a real home, getting a consistent diet and medical attention as needed. It can be really harsh outside.. There are cars, poisonous substances, dogs, people with their hearts in the wrong places, parasites such as worms, fleas, ticks, contagious illnesses, etc.
If she is completely feral, as in, she has no hope of ever coming close to a human, no matter how long you've been feeding her, then you may be able to get help from a trap/neuter/return program. They may be able to help spay her and provide food. Otherwise Special Kitty can be had for as little as $3.37 for a few pounds from Wal-mart. Feed her some bland scraps of meat until you can get to a store. Unfortunately the samples are small and you are only likely to get one per company. If you get 15 companies to send a free sample, you will have perhaps 2 or 3 weeks of food. Then what? Plus randomly changing the cat's diet every day can be hard on their stomach. I'm not trying to discourage you, just encouraging you to think outside of the box. You will get more information in here about free samples.Good luck!