I have a few places locally where I can get them from, but I just buy the 100% Human European weaves, cut them down to size and sew the clips on myself, and I sew 2 rows of weave to each section of extension if that makes sense? Altogether when I do mine it costs me around £40 for 18 inch, but it does take a fair bit of time (and sore fingers) to do them
I have 2 longer ones at the bottom (using 3 clips) then 2 that have 2 clips on above that (level with the tops of my ears side my side) and then 5 the size of 1 clip that I use towards the front of my head.
If it helps I'll post some pics of what I mean