If you don't already own the first 2 seasons they aren't that expensive on Amazon.
A fellow mod over at r/startrekgifs has compiled a good page of helpful tools and resources (ignore the script search part, that is Star Trek specific).
So if you don't coincidentally have any editing software like After Effects you might want to start out by using some of the simple online tools like ezgif. Look for HD Disney clips on youtube as source. Maybe start with a simple reactiongif without any text.