Not sure I understand. You want your boobs to look bigger but you DON'T want cleavage? Why? You mean like you'll be wearing a shirt with a high neck and you just want a bigger boob-bulge? What size breasts do you have now and how big would you want them to look?
getting a bra with a but of passing can help make them look fuller AND prevent people from seeing nips through your shirt :D getting a bra with a slight lift can also help, it also makes me feel like mine are being help up properly even after the bra is getting older
Victoria secret has a bra called the Bombshell it will increase you two cup sizes and works wonders. They carry them in all different sizes too. It is a bit expensive but its worth it so go in and ask to try on the bombshell and to be measured. Let me know if I can help you more.
Like scnbabe89 mentioned, the bombshell bra adds two cup sizes. However, I wouldn't recommend that one, it's just TOO fake. It's called the "wonder" bra because when the bra comes off, the guy wonders where your boobs went. Lol I really like the Victorias Secret Fabulous bra (I think that's the name if it). It's amazing but it's still natural looking. I'm an A cup and this bra looks great and makes your breasts appear fuller. It's also very comfortable.