Have you looked for tiling software ....
STile is a free program that tiles images.
http://www.harmware.net/(Harm is the name of the guy who made the Stile software, so don't worry about the name of the site)
You could make you own tile pattern, and then upload the tile pattern to zazzle.
I have been using it forever. You just need to save your work in .bmp - because otherwise the program makes the dpi too low.
Then change it to jpg or png in another program.
I know there are other tiling software out there, but this is the one I use.
Donna