For my 2001 E53 3.0i,
the front windows, left and right sides, each take two black plastic window clips to connect the window to the regulator slider. Part number 51-33-8-254-781
https://www.rmeuropean.com/Products/...4781-MFG9.aspx
(you can enter your year and model there and it will tell you if it fits)
The rear windows, left and right sides each take one white/natural plastic clip to connect the window to the regulator slider. Part number 51-35-7-011-680
https://www.rmeuropean.com/Products/...1680-MFG9.aspx
Current prices for the genuine BMW parts there are $2.01 and $9.73
If you search this forum with those part numbers I bet you'll find a lot of other good info.