I just found out that Humble Bundle has a book bundle for Terry Pratchett’s Discworld. A 39 book bundle that is redeemed through Kobo.com.
Edit: Only available in the US.
https://www.humblebundle.com/books/terry-pratchetts-discworld-harpercollins-books
Otherwise I will continue to slowly accumulate them through used bookstores.
Buy your legal license, then download it from the high seas sans drms.
I’ve been able to strip the Kobo DRM out of a couple of book bundles using Calibre. Haven’t bought this one yet, but I’d assume there wouldn’t be a problem.