Holy Mackerel, I just stumbled onto the website of the Chris Beetles Gallery which has a ton of original art for sale by great artists including Quentin Blake (above) and Ronald Searle! Be forewarned that the prices are pretty steep. I can't afford them myself, but you know, my birthday is coming (hint, hint).
UPDATE: Oh and they have E.H. Shepard originals too. You can own this beauty for only $135,000.00. C'mon, look under those couch cushions, people!