We started with Gerber Gentle when supplementing with all 3 of my kids, it is supposedly broken down a bit so gentler on their stomach similar (but obviously not the same) as breastmilk. I am still supplementing with DS3 and I switched to a generic gentle formula when he was around 6 months when my supply really tanked because it is about $10 cheaper than Gerber.
When we started supplementing we had lots of similac the hospital gave us. It gave LO terrible gas, so I started going through the samples I had received and we settled on Enfamil. We are only supplementing about 6 ounces a day, so we have not switched to a generic.
We've used the generic Walmart Parent's Choice Premium formula since our daughter was 8 weeks old. She has had no problems with it (she's almost 9 months now) and it's very affordable. Since your baby is exclusively breast-fed, I would say try a small can of a Premium/Regular blend formula. If she doesn't seem to respond well to a Premium/Regular blend, you could always switch to a Gentle/Sensitive formula. But when in doubt, talk to your pediatrician!
Re: Choosing a formula
This, we are switching to generic at month 7.
Henry Cavill...You're welcome!
BFP #3: EDD 1/10/13 **DS born 12/30/12!!!**
BFP #2: MC 7/2/11 @ 12 weeks
**Missing our February '12 LoveBug**
BFP #1: MMC discovered on 12/6/10