any hardware store should have them. If you're putting them into an all wood ski just use square-drive 5,5X9,5 or 5,5X11,5 funnel head screws (also known as alpine screws) for the front and front of the heel piece, and 5,5X21,5 for the rear of the heel. Drill 7.5 to 9mm deep with a 3,5mm bit (4,1 if into metal). Goodluck.
Or just take them to a ski shop and they will do it. They should stock screws from SVST and Wintersteiger too.
Hope this helps.