Seed of Issac is definitely out of print - before Rex's death about a year or so ago I was in contact with him and he said he had then no intention to reprint it as there were many mistakes that needed correcting (some of which were corrected in his later MacIssacs book) - I also spoke to a number of 2nd hand dealers at that time and they had no copies though I did manage to get one for a friend.
If you want a copy I can keep an eye out for you but must admit I havnt seen a copy for some time though at one time it was readily available.