Old monitors and TV's are pretty readily available these days.... just ask a few friends, post it on your facebook page, look out for one on freecycle
... and Goodwill always has a stash!
I however, simply walk up to my attic & select...
(The neighbors all know I'm crazy, so they bring me their trash ).
Cats seem to like to be "cozy"... pick a monitor, that's not to big, and not to small. I generally use monitors between 15 & 19 inches.