How Do I Find My Family or Clan Tartan? And what is a "Sept"?

How do you find a particular family tartan if you are not seeing a tartan specifically for the name but the website says the name is associated with some other clan's tartan?

Tartans:
Rocky: MacDonald Clanranald Weathered
Erik: Alaska State Seal
Ian: Appalachian Folklore

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Started in 2003, USA Kilts is a team of kilt makers and artisans located in Spring City PA. In 2017, we made it our mission to build community and bring kilts and Celtic culture to the forefront with our YT show: Kilts & Culture.

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