Substitution for green meat radish
I'm in the US and I had never heard of a green meat radish. However, based on information I found on Specialty Produce, I would think you could substitute almost any regular variety of radish you find at your local grocer.
Excerpts from the linked page:

Description/Taste
Green Meat radishes are easily distinguished by their shape and color. The swollen and elongated taproot is two-toned like several radish varieties, yet it is unique in its coloring. Its upper half near the stem end is lime green colored, and its tapered lower half is cream colored. It can be harvested when as small as five inches or as large as ten inches. Its thick skin covers a green to creamy white flesh which offers a crisp texture and a radish flavor that can vary from mild to hot depending upon growing conditions and maturity.

and

Applications
The spicy flavor and crisp texture of the Green Meat radish shines in fresh preparations. Slice thin and add to sandwiches, salads or wraps. Use to add a spicy accent to tacos, nachos, and Mexican soups. Slice lengthwise and pair with cream based dips or soft cheeses. Grate and serve as a condiment with sushi or sashimi or add to slaws to give them a spicy kick. In China Green Meat radishes are popularly pickled along with Sichuan peppers. Green Meat radish greens can be added to soups and stir-fries. To store, keep Green Meat radishes refrigerated and used within one to two weeks.

More interesting reading on the linked page, also.