I have the same problem with a Marstrom and my home built.

There are stainless metric wrenchs available but they are not cheap. You are looking at $50-80.

For high quality, try:
http://www.steritool.com/tools/open_end_wrench.htm

China is starting to sell Cu-Be and soft stainless wrenchs
http://www.aliexpress.com/store/200...rench-DIN-Standard-Aluminium-Bronze.html

These are 304 SS which are really too soft for wrenchs and Cu-Be is toxic.

You can also try eBay for direct sales from China


I personally use some SK wrenches from the local auto parts store, marked "Rust resistant, triple plated" that I sprayed with bake on type cold galvenize. They rust a little They rust a little but, I will probably lose it over the side before it becomes unusable.