The parts for these vehicles are not that hard to come by on the internet - there are several sites dedicated to the Mustang II.
The price will be significantly more for a 302 V8 than for the V6 or L4 engine - and the overall condition will be very important also.
I owned a '78 Cobra II for many years and I thoroughly enjoyed it - in stock form, it was never a muscle car by any stretch, but it is easily modified for much better performance. The Mustang II or Cobra II does not, however, carry the premium price that earlier models do, for what ever reasons.
I sold mine about four years ago for $5600. The new owner resold it a year ago on EBay for $6500.
Your best bet is eBay, A friend sold a King Cobra II in great shape last year for $2900, the buyer got a great deal !
There is a great group on yahoo devoted to them.. http://www.mustangii.org/exchange/