Try these resources:

Celtic Art Coracle

Hundreds of examples

Draw Your Own Celtic Knotwork

How to Draw Celtic Knots

Aon Celtic Art

History and meaning in celtic art

Eight Basic Knotwork Patterns

From Celtic Art in Pagan and Christian Times, by J. Romilly Allen, c1993.

Artwork from the Book of Kells

from the Celtic Digital Library

Google Image Search

Try "Celtic knots" or "Celtic knotwork"