The tune of "Wenceslaus" is "Tempus adest floridum" from the Piae Cantiones of 1582. I don't have a copy of the Piae Cantiones, but it ought to be on the internet somewhere; you'll presumably find the text there.

Note that a 1582 date is not medieval. Nor is 1561. I suppose you could argue that that's nitpickery, but it isn't -- the point I'm trying to make is that you are unlikely to find the text in a book of medieval lyrics. It would most likely be in a book of Renaissance lyrics.