Why is that even with 300 four star search results for the one file, it says ".... files tried" up to 300 and then says "need more sources"?

Those are generally spam / fake files.

1. Beware of Fake files in search results

2. How to find music which can still apply in principle to other types of media.

3. (a) Problems with the search, __ (b) Hostiles List

4. Ignore files less than 2,000 KB (2 MB) as most are spam & many viruses, etc. Arrange search results by size (click on link)

5. You can use the junk filter to designate files less than 1,000 KB as junk. Select those results in search results & click the trash can icon. Also go to Tools menu > Options > Junk & set them to show at bottom of results.

They don't appear to be junk - the file is 350MB which is how big the file should be.

Yes ... but may be "FAKE". Else there's not many who have the whole file.
Fake files are as though the people who set them up have upload slots set to zero. So you find them but can never download anything from them.

Best way of searching for hosts is to search the file the same way you did originally.