The following recommendation is from the Product Manager. The closest alternative to D300A-AT would be the 4Ghz probe D400A-AT-PB2, Manufacturers Product Page - Teledyne LeCroy - D400A-AT-PB2.
The part is not in our system and could be requested by filling out this Special Order Product Request Form (digikey.com).
The TechForum Cross Reference category is for parts that Digi-Key does not sell or parts that have gone obsolete. Our Engineers and Techs have reviewed the specifications for these parts and made a recommendation for a replacement. Please review the suggested part to ensure it will work for your application.