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Do you mean collarless as in a t-shirt style rounded collar, or a business-style shirt that simply doesn't have the turndown part of the collar, like the ones clergy and some of the UK lawyers and businessmen still attach the separate collar? If the latter, try "banded collar."

For example, here's an Irish site that has a sale going on that has the banded collar shirts in black:
http://www.movilleclothing.com/" target="_blank