You may be out of luck. Read the following article:

http://www.gardenwiseonline.ca/gw/sustainable-gardening/2007/07/01/walla-walla-onion

Seems sets are in short supply and only in Spring. I was looking for seeds about a month ago and there were none to be had so it's probably too late to find any now. Good luck!!

I buy my favorite onions at the supermarket. Cut off the root end and plant them outside, moderately deep and then next year collect the seed. I do this for my amusement because I am first a tinkerer. This may not be the best way. It may not even be one of the better ways. I find it a most interesting way.