SA sets are generally limited runs. The two I have were made once, and only once (Nuclear Data Green and Hack'd by Geeks).
Although there's a lot more SA sets around these days. If you watch classifieds you might find a set of two turning up for sale from time to time.
Thanks, yeah I've been keeping my eye out.
It seems there's enough demand for them to be in constant production though it could be because they're rare that there is that demand?
It's more a case of the cost for a small run being a bit excessive. Group buys work because lots of people want to buy the sets, so the manufacturer can setup their equipment and get the right coloured plastic in and do one large run, rather than setting everything up for just a few sets.
Most people who buy in are unlikely to buy a second set, so some time has to pass until there is enough interest to do another big run.
Sometimes Pimp My Keyboard will make some extra sets and sell them in their online store.