Unfortunately, you're fighting an uphill battle on this one. Dingwall sixers are rare enough as it is, and they come up in the used market very infrequently. It took me a few years (no joke) until I could actually get my hands on one.
I've seen exactly 1 used ABZ6 come up for sale (beside the one currently at Bass Direct), and that one was also in Europe. Used ABI or ABII sixers seem to come up at an average rate of about 1 or 2 a year, but they also disappear pretty quickly.
Currently, I know of two advertised new sixers in the North American market:
1) A natural ABI at Bass San Diego - [url:dv50srry]http://www.basssandiego.com/dingwallbass.html[/url:dv50srry]
2) A trans black ABI at The Low End, but I can't say for sure that it's still for sale - [url:dv50srry]http://www.thelowend.net/gallery/viewtopic.php?t=5946[/url:dv50srry]
Forum member [b:dv50srry]cheamo[/b:dv50srry] also has a 6 string Z1 available for $3500, but I don't think he's looking for trades - [url:dv50srry]http://www.talkbass.com/forum/f126/fs-dingwall-z1-6-string-spalted-maple-$3500-890994/[/url:dv50srry]
Can you tell I've spent a bit too much time keeping tabs on these things? Heh...
Honestly, I feel like your best bet might be to bite the bullet and order a new ABZ6. That's what I was preparing to do before I snagged an ABI. The price difference between a used ABI (which tend to go for around $1800-1900USD, after which you have to factor in shipping and customs charges) and a new ABZ6 isn't all that much, and it might save you a lot of compulsive checking of the forums/eBay/Craigsjiji, etc.
I'll let you know if I stumble upon anything else, though.