Straight up knots, borders, corner pieces, or all of the above? If you can lay hands on Moongate Design books (google when in doubt), they have some really nice and basic patterns in one of their books.
I have a few, I can try to find, but all the ones I have were free patterns from americanwhimsey.com. They also have a book of patterns you can buy and they can either mail it to you or you can download it from your computer
Let me know if you want me to try and find the ones I have (it may take a couple of days)
Hey Annabelle, I was just looking through the old postings in Culture and Heritage and I found a fposting that CelticRose had started called Celtic Art. She listed three web sites that had celtic art and I checked one out and found that it has celtic cross stitches. The site is http://www.novareinna.com/constellation/celtic.html Hope this helps you out!!