They have breast shaped nipples on certain bottles, great for the transition or even combining with pumped milk and regular breast feedings.
Oh, and some babies have no trouble going to a bottle. My son sometimes breastfed and sometimes had pumped milk from a bottle and never had trouble with all the switching.
As long as you can get them to latch on correctly there is not alot of pain and discomfort. If they don't latch correctly that's when you have trouble. But there are nipple shields that I hear really help to toughen up your nipples and get them used to latching. And get some lanolin and use it every time. Don't wait until your cracked and sore to start using it.
I just had my son almost 3 weeks ago and he wouldn't latch correctly so he tore my nipples apart so much that feeding him was the last thing I wanted to do. Now I am pumping and gojng to switch over to formula.
You can use a nipple shield. Just make sure you get the right size!! I also like the Medela brand lanolin. Totally helped any soreness.
I breastfed my third child for three months. I knew iwouldnt do it long so i always subbed with formula and bottles. He had no prob or confusion. I never had and pain but there are creams out there that you can buy to help with soreness. Hope this helps.