The smallest I've seen is a 5 gall and with pump, filter and heater works out about £20 Why don't you look in your local paper for a second hand one. Often there are some there, complete with everything, going for a bargain price.
ive seen people use buckets not shure if i would better to get a 1' tank and they normally go for less than 10 dollars glass so they last longer. also remeber you dont need to be able to see them so i suppose you could use most containers so long as you look after the water chemistry right. box filters are a good idea as they are not as powerfull as the other types of filters.
guppieluver, you always need a filter or else the fish are in stagnant water. and for fry you the water as good as possible so the largest percentage will live.