Have you tried eBay?
I don't remember seeing much online as far as solid colours when I was looking at yukatas, but I wound up just making mine. It's a pretty easy pattern if you can sew (or have someone you can bribe into sewing). If you can't find yourself a grey one, especially in the shade you want, you could always look for a white or off-white one and dye it -- that might be easier.
Wish I could've been more help. Sorry!