Biovea UK is considered to be good, but expensive, lots more available online to choose from, I get mine from overseas, but it's best not to mention suppliers in the open forums as they are always a target for threats and attacks.
Just to make sure you are aware that here in the UK our docs strongly advise against using oestrogen directly on nipple/breasts due to a potential increased risk of breast cancer. Admittedly, the research UK docs go off is donkey's years old and only sampled, from memory, post-menopause ladies. Neither has there been any studies/research on men and oestrogen application to breasts.
As we have no hard factual evidence I tend to be guide by our lovely queen of breast growth Lotus who suggests alternating oestrogen and prog cream daily, so that is what I do.
Albeit, I'm chasing lower-half feminisation over encouraging breast growth so I apply my topical to butt, thighs and hips alternating each day est then prog.
However, if I was wanting breast growth I'd apply it directly just as many on here do, so ultimately, it's about doing the research, knowing the risks, and deciding for yourself.