I like all things medieval, and I think medieval designs are a great source of inspiration for parchment patterns.
This gorgeous design came from this week's free samples from Dover Publications, part of the Medieval Designs book (the downloads expire so it may not be available when you're reading this).
You download it in full glorious colour, but to make it suitable for tracing just use an image editor to convert it to black and white/pencil sketch, using the free Picasa software for example.
You might need to play around with the contrast etc to get a really good traceable pattern.
This picture doesn't do it justice, but when you see it full size it's really intricate and perfect for parchment!