Alright, thank you guys :)
diachong is right, there's not a lot you can do... there are places you can go to find less expensive bras... some higher end consignment shops, etc sell good bras at very reasonable prices or larger discount stores are less expensive than some others. There are some exercises you can do keep the muscles stronger, but you'll still lose size if you keep losing weight.
I don't think so. Breasts are mostly made up of fat, so when you're losing weight that is one of the places where you will lose weight. Everyone is different, I don't really lose fat in my breasts when I lose weight, but apparently you do. There's really nothing you can do to stop it except for stop losing weight.