As with any claim like this you have to look at the primary sources.
When it comes to the more recent versions of the Nazi saucer claims (ie. leaving the mystical Vril stuff out and concentrating on Die Glocke), the only source is the Polish author Igor Witkowski, who claims to have been shown by someone (he won't say who) a transcript of the interrogation of an SS officer who was protecting flying saucer research. Of course no one else has seen the alleged transcript, so it's hard to call that evidence for anything. UFO researchers eventually went to the site Witkowski gave for where the research was supposed to have been conducted, but the "mysterious structures" they found were easily shown to be mundane building remains, even by other UFO researchers.
The idea that the Nazi have UFO bases inside the hollow earth also comes from a single source -- the alleged "Secret Diary" of Admiral Byrd. The diary, which showed up as a mail order item available from "The Society for a Complete Earth" sometime in the 1970s, claimed that Admiral Byrd had flown to the interior of the earth via the North Pole and encountered crystal cities full of UFOs and Aryans. Again, this single source is hardly credible and is contradicted by pretty much the entire historical record, but some UFO and hollow earth researchers like to claim that the diary is genuine, and Byrd, despite keeping it totally secret until after his death, changed the dates, maybe the places, and really meant Nazis when he said Aryans. In other words if you change every single thing the diary says, it's completely true. These kinds of researchers always wonder why they aren't taken seriously. You can read the whole saga of the diary from the believers perspective here, or in the book Admiral Byrd's Secret Journey Beyond the Poles by Tim Swartz.
The strain of Nazi UFO-ism that talks about Vril and Haunebu saucers, I'm not really sure where it comes from, but I didn't really hear about it until the internet. I suspect the website http://discaircraft.greyfalcon.us/ is the source, but I can't prove that. The author of that site (he calls it a book) doesn't really explain where any of his information comes from, and his odd tendency to sprinkle pictures of naked women into discussions of aircraft engineering has always made me wonder if the site is entirely serious. Still, I've never found any detailed information on the alleged Haunebu or Vril saucers that predates it.