There are a spectrum of solutions to creating anonymity. If your source is just a faceless worker who happened to overhear something about their boss, then all they need from you is a burner phone, an email address on a public server, and GPG keys—all of which you can give out the public halves of.
If your source is Edward Snowden, and his information is about the state, then I would suggest that there's really no way to do things other than full-on "witness protection" in the sense Snowden achieved: the state is going to figure out who leaked the information, so even without giving other journalists access to your source, anonymity for them (where they currently live) is no longer possible. You may as well fly them to Russia; at which point they're safe enough that you can just give them the a public email address, tell them to access it from an Internet cafe over Tor, and now they've once again got a contact point that anyone else could interact with them through without "blowing their cover." So why not give it out?
But then how does that prove anything? I can give you an email address and phone number of someone who will tell you anything I want.
Which is quite unrealistic unless you're talking about blowing a story about the local bakery or something like that. If any journalist can access the witness, they have no real security. Journalists are not all incorruptible angels, and a whistleblower is already in danger by trusting just one or two.
Witness protection works by essentially making the witness and their immediately family cut off everyone they know and restart their lives with new identities. It's a tremendous cost to pay, and the only reason most do it is because they're also implicated, and want to cut a deal. Who in their right mind would completely ruin their good, law-abiding life just to report some corrupt politician?