I was going to suggest Old Navy + TJ Maxx which have already been suggested. Also I’ve found that getting a real sports bra sized in bra sizes (not a sports bra in s, m, l) has been really helpful.

And re: Lululemon, I love my Lululemon stuff, but I wouldn’t recommend starting there first because once you go Lulu, it’s hard to go back to cheaper workout clothes. I’ve been trying to wean myself off a Lululemon addiction for about 6 years with only moderate success.