A lot of these stories are on the Internet - however downloading them can cause legal issues as Terry obviously isn't getting any royalties for downloaded stories.
I don't know if you've managed to read The Sea and Small Fishes? This is a lovely story about Granny and the witches at a Witch Trial. Well worth a read if you can get hold of it.
There was a book published that contained just about all the short stories -
Once More With Footnotes, but it was expensive and a short run. I just looked at Amazon.uk and they want over £500 for it.