There are a few options to do macro on the cheap.

• The most common is extension tubes which are hollow tubes that basically just move the lens further away, which decreases the minimum focusing distance.

  If you're really stuck you can just hold your lens in front of the camera. Focus and composition are a bit hit and miss with this method! For more information, see Freelensing! Turn any Lens into a Tilt-Shift or Macro

• Another option is to use close up filters, which are additional screw on optics that go in front of a lens to allow close-up focusing. I haven't had any experience myself, and it's not the cheapest option. For a good low-down on close up filters, you can read more at Comparison of Close-up Filters and Macro Lenses

• Finally, you can mount a telephoto lens in reverse! This can be the cheapest option of all (besides freelensing) because you can make a reversal ring from a pair of lenses and body caps. However, with your 18-135 lacking manual aperture control, you'll be stuck shooting wide open.

If I were you, I'd pick up a set of cheap extension tubes and go from there! As for photographing single snowflakes, I admire people with ambitious goals, but I think you might be reaching too far here because you've chosen something seasoned photographers with many years macro experience would struggle with.

Typical macro lenses provide approximately 1:1 magnification, meaning the image on the sensor is life size. The sensor in your camera is about 22mm across so that's effectively the smallest thing you can image. To fill the frame with a snowflake you need to go supermacro, which as Shizam states is beyond these budget macro options. Depth of field is so small (like a hairs width) at these distances you need a very precise means to maneuver the snowflake into shot.

You can search YouTube for videos of people capturing snowflakes to see how they did it.