Here are some ways I can think of to find carol texts.
There’s a site called Public Domain Poetry with a big database of public domain poems. The search feature there seems to be broken, but Google can do the job. Here are some Google results, all within Public Domain Poetry:
You could also re-set a choral piece that’s in the public domain. The Choral Public Domain Library has a huge database of public domain sheet music and text. Here are some CPDL search results: