My friend just went through this, her LO eventually took a beaker around 8 months, but I honestly think throughout the whole time she was spending money on bottle after bottle that the problem lay with not laying the law down enough, rather than the right teat etc.
Having said that, my friends and I have all found the only bottle our LO's didn't reject at any point was the Tommee Tippee "Closer to nature" as they're very breast like.
Also, Aptamil is the most recommended formula, as it taste's the most like breastmilk, which I checked myself! It's sort of nutty and sweet unlike SMA which was bitter and horrid. It also has lots of Pre and Pro-biotics.
Maybe get the bottle I mentioned and then prepare to be tough.
Agree with LadyBee's point about whether or not she knows what's it in is useful before this, but assuming she does... and isn't underweight or anything, here's some of my ideas:
-I'd start with expressed milk maybe, then move to a mixture of expressed and Aptamil, so it's not too many changes at once.
-Get her dad to do it, and don't be in the room because they can smell you a mile off.
-If you're strong enough to make one big offer to nail it, you could try offering her nothing but a bottle for a whole day. Sooo hard to actually put in to practice when every hormone in you body and your heart are telling you to give in though!
-If not then perhaps choose one feed a day to try it at, and if she doesn't take it she just has to wait until her next boob feed. Only takes a few days IMO for them to respond to routine changes. She might then take it if she doesn't feel it's going to threaten all her feeds.
My friend found this the hardest thing which is why I don't think she succeeded, she'd give it 5 mins of her LO squirming away from bottle and then promptly get her boob out.
Sounds harsh but as my mum says, (amusingly about dogs OR children who won't eat their dinner) make it clear that's what they're getting and if they're hungry they'll eventually eat!
Again though, must stress that baby needs to be thriving/old enough before trying withholding feeds. Sure yours is though.
-If none of that works then last idea would be to give her boob, then offer a top-off bottle feed once she's had her comfort and doesn't feel it's threatening the boob, but an extra treat?
Wish you lots of luck
x