My Pat 12 had a mixture of pan head and hex head screws for engine compartment and sheet metal. Last time I looked for the screws large pan head slotted head I got them through Fastenal
http://www.fastenal.com/web/home.ex I'll try and find the part number. Long and the short of it was bags of 100 cost what hardware store charges for 20-30.
Cheers Phil