If you aren’t lucky enough to live in or visit one of the countries (e.g. Canada, you can search for Faded Page and/or Project Gutenberg Canada) where all of Margery Allingham’s books are in the public domain, you’ll be happy right now because several of her Albert Campion books are $1.99 right now in the US. And they are discountable at Kobo US!
Sweet Danger, #5
Buy at Kindle US | Kobo US
Flowers for the Judge, #7
Buy at Kindle US | Kobo US
Traitor’s Purse, #11
Buy at Kindle US | Kobo US
Coroner’s Pidgin, #12
Buy at Kindle US | Kobo US
The Tiger in the Smoke, #14
Buy at Kindle US | Kobo US
Hide My Eyes, #16
Buy at Kindle US | Kobo US
Note that Allingham’s books do go on sale pretty often, so if you want the other series titles, keep an eye out here, and I’ll post them when I see them. Or if you see one or more on sale yourself, please add the info in the comments!