Farsi, if memory serves, was supported in WorldScript in around 1990. I am not sure if you are looking for support for the specific style favoured by speakers of Farsi and Urdu, that is, Nastaliq, or if you are looking for support for the Farsi writing system within the Arabic script.
If you are looking for a Nastaliq, then this is supported in TrueType 2 / TrueType GX (: through cross-stream kerning), but there are no shipping Nastaliq fonts for TT2 as the number of ligatures makes this style of the Arabic expensive to design and develop. Monotype has updated its Nastaliq, introduced in 1981, which is now available in OpenType.
If you are looking for support for the Farsi writing system, then begin by opening System Preferences in Mac OS X 10.4 or higher, then opening International, and in the Input Menu select Persion. There are several TrueType 2 fonts for the Arabic script in Mac OS X, and they should work for you although they are in the Naskh style.
Below on Persian language support in Mac OS 9 which includes several fonts. One, if I remember rightly, is Mashad. These fonts should still work, although I am not sure:
TITLE: Mac OS 9: About Persian Language Support
Article ID: 60467
Created: 9/27/99
Modified: 2/2/00
TOPIC: This article contains the Read Me document for the Persian Language Support feature of Mac OS 9. The Persian Language Kit Support disk image is included on the Mac OS 9 disc in the CD Extras folder, in the Language Kit CD Extras Folder.
Note: The Persian Language Kit requires a component of Mac OS called WorldScript I. The Read Me
below does not mention this. Before following the installation steps provided below, use the Custom
Install feature of your Mac OS 9 install CD to install WorldScript I. Select Customized Installation for
Mac OS, and click the International checkbox to select it.
Best wishes,
Henrik