I swear Xûr isn't going to sell them any time soon.
I haven't seen a boot engram drop from ToC in eons.
If Xûr does sell a boot engram, it's a legacy engram.
Nightfalls pretty much only reward crap legendaries now.
Pretty sure these are the most difficult exotic to acquire.
[i]~Nyahh! =^,.,^=[/i]
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A clanmate didn't grab them when Xur sold them. He's been hunting for them ever since. Not a single boot engram decrypted into TV. They're probably bugged, at least in engrams.