Check ebay and amazon and even browse through various ant forums, as there are a number of individuals who sell a couple of nests here and there without an actual website. I have noticed that the different ebays (ebay.ca and ebay.com) often carry different things.
I have sold some spare nests here and there.
The main groups mentioned in the US are usually AntsCanada, TarHeelAnts, and ByFormica. There are a few other individuals that sell quite a few, but they are quite scattered around the internet on ebay, amazon, craigslist, kijiji, etc.
They don't have a whole lot more than accessories and small formicaries right now, but I hear they're coming out with a really nice formicarium in the next few weeks.
Terry, when referring to "they" are you referring to ByFormica or http://www.antkit.uk/?