I'd never ever plant my own - or supermarket spuds. For what they cost, it's really not worth it and I think irresponsible. OK 99.9% of the time you'd probably get away with it, but it only takes that 0.1%.
As for buying them, there's a nursery just down the road from us that I can get them from. They don't have the choice that the nursery that closed had, but they have the ones I'd choose.
I'm dropping Casablanca next year as they all fell apart - even when steamed, so it's the good old Lady Christl, Charlotte, Kestrel and Ambo.