Re: TEK514 needs new 5V4G tube
If you are hesitant to use eBay you can buy a NOS tube from Antique Electronics Supply.
They currently list a 5V4GA for $29.90 at https://www.tubesandmore.com/products/5v4ga-rectifier-full-wave.
If you can find a 5V4G in stock that version usually sells for a much lower price.
Note that, per the RCA tube manual, the 5V4G and the 5V4GA have the same electrical specs but he 5V4G is packaged in a larger old style bottle. If you fine one, you will need to determine if it will physically fit your scope.