My lactation consultant recommends only the Second Nature nipple, which fits on either Second Nature bottles or Avent bottles.
Here is a link that describes the nipple:
http://www.regallager.com/products/sn/feeding.aspx
We have used this nipple for our exclusively BF 5.5 month old son and have had no problems. The only "drawback" is that the baby has to work to get the milk out of the bottle (just like he/she does at the breast). The milk will not pour out like a regular bottle nipple. What this means is that your baby cannot get lazy with the Second Nature nipple, or the milk will stop flowing. Tell your alternate caregiver to expect this. It can take at least twice as long to bottle feed using the Second Nature nipple but it is worth it, because this nipple really is like mama's natural nipple, and your baby is less likely to experience nipple confusion and start nursing differently (i.e., painfully) at the breast.
Here's a VERY important tip. A lot of grandmas and alternate care givers are used to bottle nipples where you give only a half inch nubbin to the baby and the milk pours almost effortlessly into the babies mouth. You CANNOT do this with the Second Nature nipple, or you will end up with a very frustrated baby (and the caregiver will announce that the nipples don't work). You MUST make baby open his/her mouth VERY wide, just like at the breast. Baby must have his/her mouth surrounding the nipple with flanged lips, just like at the breast. Baby must suck the Second Nature nipple just like when he/she is at mama's breast.
One last thing. The Second Nature nipple is NOT a newborn nipple. It is an all stages nipple (unless you get the preemie nipple, which I have never seen). If someone tells you that you will need to get a nipple with larger holes as your baby grows, you can tell them that your nipples have not changed since his birth and the Second Nature nipple mimics your own nipples.
I hope this is helpful to you!