You're not the only one. Lots and lots of women complain that workout clothes, running clothes in particular, are hard to find in larger sizes. Heck, even I have a hard time finding tops that fit, and I'm not that big! I'm pretty chesty; apparently runners are supposed to have mosquito bites, not half-gallon jugs.
Did you look in the men's section? My fiance loves the C9 brand running clothes at Target. They're not very expensive, and you'd be more likely to find loose-fitting tops there. The colors aren't very exciting, but fit is more important.
As for the BMI thing, it's really hit-and-miss. It doesn't take into account your frame, where you carry your weight, etc. A better progress report is to measure your bust, waist, and hips (and thighs/upper arms if you want) and keep track of how much smaller they get. Muscle is denser than fat, so as you tone up you may not lose much poundage, but your overall size will shrink.
Good for you wanting to get healthier! I believe that everything we do--yes, even quilting--is more enjoyable when we've got a healthy body. :)