Too many shops to mention individually, but plenty round Times Square and 5th Avenue area around Empire State Building, also Little Italy, ooh, and there were some good ones on 8th Avenue round W44th Street. Basically, you can't miss 'em!
Definitely around Times Square & 5th Avenue - the place is packed with shops selling naff pens/models/t-shirts etc They're not exactly cheap, but they're definitely tacky :)