P.,
That's a very personal decision. Kudos for nursing exclusively for so long. I'm not sure of the reasons you have decided to supplement with formula with this child vs. your first one, but have you considered continuing to pump so you can still feed her breast milk during those not from mommy feedings?
What ever your reasons may be, there are many formula options out there and while all of them will claim to be the best, it's best to remember that you will be going from feeding a living food to feeding her a dead, artificially fortified one.
If you're interested in convenience (powdered formula) my strongest, and only recommendation is for Baby's Only from Nature's One <http://www.naturesone.com/index.php> They have a milk one with some soy and an all soy formula. Both are organic, non GMO (very important) and of the highest quality. I used this brand with my son occasionally when I had to supplement for medical reasons and the supplementation time wasn't long enough to merit obtaining donated breast milk through Milk Share <http://milkshare.birthingforlife.com/>, or buying the necessary ingredients for the best formula option of homemade <http://www.westonaprice.org/children/recipes.html>.
And as someone mentioned, whole milk at this age is also an option. I strongly recommend goat instead of cow (it most closely mimics breast milk) and caution against straight soy milk since it mimics estrogen in the body and not exactly in the right proportions as it would be from your milk. I also recommend RAW milk over pasteurized as well since most sensitivities to milk come from the pasteurization process than from the milk itself. I'd be happy to help find you a clean source. This CAN be fed safely to anyone from infancy through old age (although I don't believe people are designed to drink milk past adolescence on a regular basis).
No matter what you supplement with I highly suggest that at 8 months, you consider a sippy cup. As you start to transition from nursing to formula/milk more and more you'll want to avoid the bottle trap that many babies can fall victim to. It can be very difficult to break a bottle habit.
I wish you luck with whatever you choose for your daughter.
Blessings,
Rolinda
Wife, Mother, Friend