We use Printing for Less to do our tracts. They have a number of formats, but the "Statement Stuffer" works best - as a good size for handing out and also for stuffing into the reply enveleopes that we get with the junk mail that comes to us. They send us junk... we reply back with the word of God.
There are other on-line printers as well... but they will give you a template for formatting your tracts... and you will have to upload the image in CMYK TIF format for color separation. We usually do 5,000 of the 3½x7-inch which comes up around $300 or so with shipping.