OK - thanks for asking.
The images for the buttons I got either from the sites themselves (e.g. http://www.stumbleupon.com/ ) or I copied off of site that were using them (see the bottom of http://news.bbc.co.uk/sport1/hi/cricket/england/8162048.stm for example - strangely the BBC and I seem to have graviated to the same buttons - though we both used to have ones with just a couple shared).
As to the code for the links, I either got this off of the bookmarking sites or by looking at how people like the BBC did it. If you click on one of the BBC sites, you can see what the URL is.
Finally, how do I get these into my blog articles? Sadly the answer is manually and it is a laborious process. Much the same as raincoaster above, I have a file with the template HTML (with all the images and links) but with the URL and title of the article in question as place-holders. Here is an example of one line from this:
I globally replace "REPLACEME" with the URL of the blog post and globally replace "BANNER" with its title. I then paste the resulting HTML into the end of the blog article.
There are applications that you can use to do the same (and do it more easily and flexibly), but not all of them are supported on WordPress.com.
I hope that helps.