It depends on the Dealer, Year, Engine, etc....
But most parts that are still avaliable are usually avaliable the next day special order. Be aware though, Ford just obsoleted a lot of parts in the last few months!
Most comon parts throughout the years that are still avaliable:
300-I6
Motorcraft Oil Filter: FL-1A / D9AZ-6731-A / Replaces: FL-1
Motorcraft Fuel Filter: FG-44-B / D3FZ-9155-B
Motorcraft Air Filter: FA-3-R / C2OZ-9601-AR
PCV-Valve: EV-76-C / D9DZ-6A666-A
Spark Plugs: SP-420 Replaces / BSF-42C
Breather Filter: (Crankcase Filter) FA-104 / DOOZ-9D697-A
Distributor Cap: Electronic Ignition: DH-367-B / E5DZ-12106-A
Rotor: DR-379-A / E6TZ-12200-D
Duraspark-II Ignition Module: Blue Grommet: DY-893 / 1U2Z-12A199-AA
302-V8
Motorcraft Oil Filter: FL-1A / D9AZ-6731-A / Replaces: FL-1
Motorcraft Fuel Filter: FG-44-B / D3FZ-9155-B
Motorcraft Air Filter: FA-972-R / D2AZ-9601-AR
PCV Valve: EV-98-B / D9ZZ-6A666-A
Spark Plugs Platnum: SP-502 / Replaces: ASF-42-P
Spark Plugs Copper: SP-450 / Replaces: ASF-42-C
Breather Filter (Crankcase Filter) : FA-675 / D8AZ-9D695-A
Distributor Cap: Electronic Ignition: DH-411-B / E6TZ-12106-A
Rotor: DR-380-A / E6TZ-12200-C
Duraspark-II Ignition Module: Blue Grommet: DY-893 / 1U2Z-12A199-AA
I could continue, but I'd be here all day writing a small book.