The best (and by best, I mean cheapest) way is to get a large bucket, fill it with water. It has to be big enough for your fermenter to sit in. Put the fermenter in it, and drape a wet tshirt over the top. The water will wick up through the shirt and cool the bucket via evaporation. If you put a fan blowing on it lightly that cools it even more. It's not a lot, but it does help keep it lower.
Oh I just saw you're using lager yeast. Oh yea, 21ºC is WAY too hot for that! BlindLemonLars is right, you gotta get lager yeast even cooler, between 9º and 15º C. Ales should always be in the range of 17º to 20º C. A fridge is really the only way to lager, and you probably don't have that. Don't worry, it'll still be good beer, just don't expect it to taste like lager. Do the above, and also freeze some water bottles and drop them in the water to cool it off more.