Not all of my work is available in all formats and all territories at all times. In general, if it’s not there, it’s not available, but a search engine is your friend here, as is my books page, which Mary and I try to keep updated with links to my available work.
That said, please do be aware that some of my books have had multiple editions and you can occasionally find paperback variants of early editions going for hundreds and hundreds of pounds on second handbook sites. Do not, under any circumstances, purchase these – unless you are specifically looking for a first edition for some reason. My books are, and should be, available for reasonable prices.