M.M.
Initially I used the 4 ounce glass bottles by Even Flo.. but I found that the baby had too much gas. My mother in law suggested that I try the Playtex Drop In system because the liner collapses as the baby drinks the formula so there is less/no air in the bottle...the bottles are plastic but the liners are BPA free..Playtex has BPA free bottles.. I've also heard that Dr. Brown bottles are very good. Vent Advanced Aire system is similar with the bent bottle.
We have used the Playtex Drop Ins and did not have any problems... I think we've had maybe one leak.. very satisfied with this product! You can get the liners at Target or CVS. Hope this helps!