Using ex-satellite dishes
Your local household waste disposal area (tip) is a great source of parabolic dishes which are very usable on the microwave amateur bands - and it's environmentally friendly to re-cycle them!
Finding the feed point
The easiest way to find the feed point is just carefully replace the LNB with the feed or replacement LNB in exactly the same position. However this is not always possible - but luckily there are a great number of pages on the web showing how to find the point:
Online calculator to find the feed point of an offset dish: http://www.satsig.net/pointing/finding-dish-offset-angle.htm
Feeds for dishes
Excellent design by Brian G4NNS: http://myweb.tiscali.co.uk/g4nns/FeedHorn.html