If you are from Costa Rica, you should definitely sublet in Chicago from December through February. It will be exotic for you!
You are going to hear a bunch of nonsense on this board because most people are agents and want to direct you to places where they can earn money.
There are always sublets in Chicago, and generally a sublet is furnished because the person wants to return to their home. Winnemac Park is a very small area, so you should consider searches that include Lincoln Square, Andersonville, and Ravenswood. You will find the most studios in Edgewater, further east, closer to the red line. About a 20 minute walk or so to Winnemac park.
And August 15th sublet will be advertising now and will probably ask for August 1st. Try to negotiate to pay half the month rent, otherwise you'll have to suck it up and pay the whole month. Same goes for October.
Use Craigslist. Despite the amount of garbage currently on there, there is no better site for the variety of subleases. You can also post an ad on craigslist stating what you are looking for and budget. The sublease world is not run by agents. It's all done by individual renters. So...use caution. Make sure the person subletting to you is the true person on their lease AND...this is important, that they have permission to sublet. There are many landlords that don't allow this.
Remember...Chicago in the winter next time. It's the best!