Depends on what you're looking for. Anything for a Mustang II is going to be a bit of a challenge, and you'll often find incorrect listings and parts that require modification before they'll work. The v6 is arguably one of the easier ones to find parts for, as the same engine was used well into the 80's in Rangers, Bronco II's, the Aerostar, and others. Finding the right parts can be hit or miss though.
Is there anything specific I could help you find? I usually try Rock Auto too, but often find whatever I can find on eBay or elsewhere. I often have much more luck using the '84 Bronco II to look up parts, same engine for the most part.