You can get them in the UK! John Lewis have them... they're pricey things tho, around £100. If you can splash the cash for one, go for a fully adjustable one in a small-ish size - it's easy enough to pad them out but you can't make a large one smaller.
If you're looking second hand, and are lucky, look out for a non-adjustable dress form, http://www.xpb66.dial.pipex.com/index.shtml
Kennett and Lindsell are the industry standard make. Again, get a smaller one and pad it out to your size, you can do more that way.
A dummy is a brilliant thing, so, so useful! It means you can really see what you're doing, take a step back and see it from a distance, get the overall look of the costume... and you can store stuff draped on it!