The certified world's champion credit-card collector _ he never leaves home without a bunch of them _ says he wants to get even more of the handy little plastics. Walter Cavanagh, ``Mr. Fantastic Plastic'' as he calls himself, says he is determined to get one of each of the 10,000 credit cards he estimates are available in this country. He figures it will take him five years to reach his goal. ``I figure I only have 12 percent of what's out there,'' he said in an interview last week. ``My goal is to get them all.'' Cavanagh, whose 1,099 credit cars earned him a paragraph in the Guinness Book of World Records, said he applies for 100 cards a month, and gets few refusals. `I did get a letter from a gas company back East asking why I wanted a card since I lived 3,000 miles from one of their stations,'' Cavanagh said. He doesn't owe on any of the cards because he pays his bills in full. He has found the only drawback to collecting plastics is the barrage of catalogs and other buy-me goodies he gets daily. ``My mailman hates me,'' he said.