The OEM belts have been discontinued by Ford, but you can probably still get some from obsolete part sources like Vintage Parts Inc. in Wisconsin.
The main drive belt is F5SZ 8620 DA, Motorcraft JK8 903. SC belt is F4SZ 8620 D, Motorcraft JK8 401. Jackshaft belt is F4SZ 8620 B, Motorcraft JK7 432 A. All of those show in stock under the OEM Ford numbers at Vintage.
cheers
Ed