I've got them from Fasttech before, but they don't seem to be available there now even if you're willing to take on post-pandemic shipping.
One thing you might try is looking for a Prime extension kit. They're $2-3 at Fasttech. You won't need the chimney piece, but the tank glass is the same as the K5 and it's still cheaper than an authentic tank glass.
ETA: The extended Prime tank glass is the same as the K5, not the regular one.