I was able to pick some of the out of print books up from Australia: https://www.gumnut.io/collections/rpgs-mutant-year-zero
The site is not set up for international shipping, but I emailed their info address and they were able to ship to me regardless. They sent me a receipt over PayPal. Note that 1: It was not inexpensive, and 2: international shipping times are killer right now. My order was in pandemic purgatory for 6 weeks.
I got Zone Compendium 3, and Genlab Alpha. M:GA appears to be sold out right now, but they mentioned being able to get more books (in general) from their distributor. As of this post some of the books are also on sale on the site.