I did my 850 sedan a few years ago, and bought all the materials and adhesive from WLS Headliners (wlsheadliners.com). I was very happy with what they provided. You might be able to find the adhesive elsewhere for less money, but I was happy buying from them to make sure I got the right stuff for minimal hassle.

One very important note: The reason the old headliner falls down is that the foam backing disintegrates with age. I found out the hard way that if you just spray on the glue and don't clean off all the old foam, the new headliner adheres to the old foam, and the old foam just disintegrates further and the new one falls down too. You can clean the old foam off the backing board pretty easily with just rubbing, so it's not hard, just very important to do!

Roger