Tube top (your choice of a bra underneath or not), crop top, halter top. Also, just going into a sports store; they usually have one or two workout tops that aren't too tight around the girls but fitted around the waist.
Halter tops *tend* to be a bit loose around the waist; but I have one like (below) that I love
its just the gym, why does it matter so much what you wear. i'd understand if you were asking about what you should wear when you go out, but all you really do at the gym is come, work out/sweat, and leave. and you dont have to go all out just to meet some guys (if thats what your intention is) i just wear yoga pants or leggings with baggy t shirts and have no problem meeting guys because its the gym, nobody expects you to look perfect and all made up.
Professional image consultants always advise either buying something and having a Tailor fix it to fit perfectly OR have something custom made by a person who can sew. Do you know anyone that can sew? Or you can take sewing classes too