I am looking to find the famous strip weher Linus comes across Snoopy freezing, and comments "Be of good cheer, Snoopy" (or something like that. Can anybody help?

I don't know where you can find that exact comic, but if you know what year it was in, it will be easy to find on Comics.com
Even if you don't know the year, you can still search for it. Here's the link: link The website reposts all the old comics, one per day. There's a search bar where you can put in the year, day and month to look for the comic. Good luck, I hope this helps.