It's not just the TO-3* package that is obsolete, so is the double-diffusion fab process that made it.
The original fabs died in the 80s, AFAIK; they've been epitaxy since then. Which means the average 2N3055 you get, will exceed its spec by 2-10 times or better, and you'll never know in which parameters until you test them a lot at a time. Or maybe you get some ancient relics!

If you must have old fashioned packages, I wouldn't recommend anything older than, say, MJ15022 or the like. If you must have BJTs, newer On Semi parts have excellent specs, and will most definitely deliver the power and SOA they claim. They aren't at all expensive, considering how much faffing with alternative sources you are saving in the process!
Even better still is using FETs in an SMPS, but that is a more advanced topic so I won't complain further in that direction.

Tim