Definitely get the slow flow nipples. Try the premie ones they may work a little slower. Give your baby time though. You are switching her from the breast to a nipple on a bottle and they can obviously tell they are different so it does take a little time to get used to it.
I used the Playtex drop ins for my 1st and now my 2nd daughter. My 1st daughter did excellent with them and rarely got gased up. Now, my 2nd daughter is doing good with the bottles, but she is getting more gassy, but she is on soy formula too compared to my 1st daughter who went on regular formula (I do supplement with formula because she eats so much).
I would honestly try the premie nipples (you can get them at walmart) and if she is spilling them still, then it's probably just a waiting process for her to get used to them.
Good luck!