Master grounds, early Tracey Ullmans and first season pieces are hard to find. Reproduction art does exist, although they do not produce too many editions. 70s, 80s and 90s is easier. You stand a good chance of getting a nice pose of your favorite character. Reproduction art is not too hard to find.
More recent items are easier.
Hard to find good Scooby Doos, Hong Kong Phooeys, and Dastardly and Mutleys. Limiteds and sericels are pretty abundant. There are some nice pieces from the 1950s around, although they are not too easy to get. Newer art is not too hard. Reproduction art - there are a few editions that the Warner Store puts out.