Ready-to-use Celtic Designs , Mallory Pearce
This is part of the Dover Clip-Art series. The book has 96 Celtic knotwork and zoomorphic designs, printed in jet-black on one side only of glossy paper.
The prints are suitable for scanning/copying for your own colouring.
The designs are copyright-free so long as you don't use more than 10 in the same project.
Other books in the series cover borders, alphabets, etc., but I don't have hand-on experience of them.
(Which would be my que to go browsing again. Most bookshops here have quite a range of such books. Cheapskate! that's me. Review without buying)
ISBN: 0-486-28986-9 Dover Publications, Mineola, NY