There is no legally available full collection. Twitch did a full run with NBC (I think it was that channel) as a stream last year and a bunch of people screen-capped it and uploaded it to a few places. I got my copy from a btsync link.
Amazon has almost all of them, all the way back to the first episode. We subscribe to PBS kids (through Amazon) for our daughter, but I'm not sure if that's required for the Mr. Rogers series.
Seasons 1-7 are available on Amazon, though I don't know if that covers all of them or not.
So does anyone know how to view/buy/rent a large (complete?) collection of all those wonderful old episodes? I would like to re-watch these with young family members. I'd happily buy a big Blu-ray or DVD set but I didn't see such a thing available on PBS' site.