@Drew51 has one or two of Burbanks whites in his collection. There was a discussion in this post Best Thornless blackberry? - #161 by Bear_with_me and there is a new cultivar on the market Polar Berry https://www.gurneys.com/product/polar-berry
polar is available much cheeper here https://www.jacksonandperkins.com/polar-berry-blackberry/p/25919/