You can buy either upholstery zips, or the popper tape here in uk, we also have several large stores who sell sheeting fabric in plain and patterened 108" wide which is ample for duvets. As for the flap to tuck under the mattress, i would sew in a piece os fabric the width of duvet by the depth of the mattress plus 6", On the edge that is going to tuck under cut a square piece out the depth of the matress, sew the 2 cut ends together and elasticate all the way along so its like a fitted bed sheet.Hope this helps