I don’t believe you can buy a disabled railcard from any third parties because they aren’t setup to handle the requirements for confirming eligibility. It was being worked on, but I’m not sure if GBR changes direction.
This is what Trainline says
Not available to buy with Trainline. We are currently working on the ability to verify eligibility. In the meantime, to buy a Disabled Persons Railcard, you can apply online here
So until it’s available from third parties no one can offer this as a perk