My mom's getting married!

She made me the maid of honor and asked me to find bridesmaid dresses which I'm totally ok with! I found some that I like and are only $50 the one girl asked if she could wear something longer and with sleeves (she's heavier and not comfortable, which I understand) the wedding is in August so I was thinking the no sleeved dress with some kind of cover so we can take it off if we get too hot but match for pictures. So my main question is, where can I find something like that? And do you think it'll look ok?