WiFi reception booster for MacBook air I live temporary in Spain. 

I have MacBook Air with OSX 10.8.

I have access to public WiFi but signal is week, and I do not want to hang over the balcony all the time :), or crack my neighbors (strong) wifi (WPA protected).

Is there a booster for WiFi signal reception, like a repeater (inverted router) or something like it.
I was thinking of a device that would connect to a wifi and then boost the same signal so I can use it inside my apartment.

My budget is limited, so i appreciate a low cost maximum effect solutions.

Any advice ?
The easiest way is to use a wifi repeater.  Many routers can be put into repeater mode.  Put the repeater on your porch.

Initially you have to attach the router to your laptop with an ethernet cable to configure it.  Directions vary between manufacturers, but in a nutshell, you want it to listen on whatever channel is the available one, then rebroadcast that on an un0ccupied channel at least 3 channel numbers away.  So if the current access point was on channel 2, you would want it to rebroadcast on 5 or above.

Google wi-fi repeater.  Try amazon.  Should be under 50 bucks.

A second solution that will work better, but is less convenient, is to put a wifi router on the porch, and put it in bridge mode, then connect to it with an ethernet cable.  This will give you full speed (repeaters cut your speed in half) and is less likely to cause/be subject to interference from other wifi points.