According to this interview it appears that Stanley Tucci was inspired in his rendition of Caesar Flickerman by Graham Norton Interview Otherwise I have yet to find a quote from Suzanne Collins on her inspiration, but the search is still underway.
From the video interview,
"When you create a character, you steal and cull from all sources. He's a cross between Wayne Newton and Jiminy Glick and there's a little piece of you Graham - the intelligence!"