Hi Steve and welcome to the site.
As you probably know, Clarecraft, the company that made the figures, no longer exists. So any figures you manage to buy now will be second hand. There was a very good Internet site for collectors that had the whole catalogue listed along with prices that they had fetched on eBay, but that does seem to exist any more either.
Keep looking on eBay is about all I can advise you. Or you may want to look at the 'Trading' forum of the Discworld Stamps Site
as the members there sometimes advertise their figures for sale there.
Hope that helps - good luck!
“Men never commit evil so fully and joyfully as when they do it for religious convictions.” – Blaise Pascal