What would be a good sized cage for a family of 4 parakeets?

I have two right now, but plan on getting more. The cage that i have now is 18''x18''x24'', but i know that isn't big enough for 4 birds, their toys and food dishes, etc. So, where can i get a much bigger cage, but not pay an arm and a leg for it? I need to buy on by this weekend because that's when i plan on getting my other two birds. please help!