Grey games boards are very cheap, you can normally pick them up for just a few bucks on ebay as few people have grey motherboards to use them with. They are much harder to find however and appear on ebay very infrequently. Also I have only ever seen the very common games like SSF2, X-Men etc for sale.
If you can pick up a grey motherboard and game cheaply then all well and good, you might even find the two for the same price as just the blue board would cost you, but if you really want to get a few cps2 boards then blue is the way to go.
Another point, you may want to read up on cps2 batteries before you decide to get into cps2 collecting. With a little bit of skill the batteries can be replaced, but it makes collecting cps2 games a tricky business.
<a href="http://cps2shock.retrogames.com/suicide.html" target="_blank">http://cps2shock.retrogames.com/suicide.html</A>
Lastly, to answer your actual question;
<a href="http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2112456565" target="_blank">http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2112456565</A>
Or failing that email bbu9715965@aol.com - he is a high volume cps2 seller on ebay and will probably have multiple spare motherboards on his hands for a reasonable price. They seem to go very cheaply on ebay these days if they sell at all and I think he's even given up listing them. I wouldn't pay more than $30 or so for one unless somebody convinced me they were going for more...