Prom dresses

My school prom is in June! The letters have recently come out and have got everyone talking. All the girls are excited and we keep discussing what kind of dress we want. I've decided I would like a maxi dress like these:





But I don't want it to be strapless because I want to enjoy my night without worrying about it slipping down. Some girls are talking about getting their custom made but that is bound to be really expensive. I don't want to spend loads of money on a dress I will probably only wear for one night. Does anyone know where I can find dresses like the above that don't cost a ridiculous amount of money?