I've recommended looking at archive.org before for Hitchhiker's resources, as they often have the tv episodes and some of the radio shows uploaded there. However, BBC Radio 4 have recently uploaded all of the radio series, Primary to Hexagonal phase! It's a really great resource when filling out info on this Wiki, but it's also great to have all of these episodes to listen to, and you can download them all for free, too!
Here's the link: https://archive.org/details/hhgttgall6
If you don't have the radio series on hand, or are missing some phases in your collection, then this is a great opportunity to save these. It's a free archive where everything is available to the public, so I thought I'd let you all know about this as it's very exciting! Sometimes things on archive.org are only available for a limited amount of time, kept up for only a few months, so check these out sooner rather than later and download them if you want to keep them.
I've downloaded them all, so if you see this later and they're no longer on archive.org, let me know and I'll upload them to a Google Drive or something so you can have access to them!
Thanks for your time, hope you're all having a good 2019.
P.S: Here's the complete TV series on archive.org too, for anyone interested: