Talk:Upgrades/@comment-112.198.118.204-20170331140739/@comment-3188353-20170331151648

You'll only have one set by the time you clear the full version of 6-4, which takes three visits once you get there, if you haven't already.

After that you can get a few more of the travel garb from clearing the newer maps in world 7 (and beyond once they're added, I imagine), a travel kit for logging in 25 days out of each month, and the letter set is only an event reward alongside the first two items as far as I am aware.