If you have small breasts you might get away without a bra but if you have larger breasts that isn't remotely practical. I have large breasts and always buy leotards that don't come low at the back for just this reason. However if you have a team leotard that you have to wear then that obviously isn't an option. (Although you might want to mention to whoever selects the leotards that it would be helpful if they took these sort of practical considerations into account.) I wear a Panache sports bra which offers great support and has a racer back option but if you don't hook the racer up it is reasonably low at the back. However sports bras are never that low at the back because they wouldn't work as well if they were. You might need to get one of those low back bra adapters. Also try to get a sports bra which is matches the colour of your leotard or is skin tone so that if it does show it is less noticeable.
You don't wear sports bras for gymnastics competitions--Your leotard should be tight enough to not show anything embarrassing. Here is where I get mine: http://www.gkelite.com/Index.aspx
Sports wear shops.
pff. really. what a waste.
