Barnes & Noble has a Celtic Design Coloring Book for $4.45. I have heard about quilters who have used designs from coloring books. I found this on pronto.com I hope it helps.
I also found a Four Heart Clover Celtic Knot as a free cross stitch pattern on
http://www.clanbadge.com/charted.htm
I didn't find any free Celtic Knot quilting patterns that are free other than the above mentioned sites.