I have some in my sashes, it's like the insert in Masona's pic, just fur on thin metal strip rather than brush. Some of the strips are stuck on, some are dovetailed into the extrusion (my sashes are aluminium) and short pieces (round the fasteners) have a small screw at each end. The furry strip is fixed to one of the sashes where the top of one passes the bottom pf the other, and also along the sliding surfaces where they meet the frame. It compresses well but needs enough of a gap to fit in. I don't know a supplier, though.