Is there another tip that can be used in place of 1M that will give you almost the same design?

You can use any really large open star tip. Or for the big swirl on top of cupcakes, I often use a very large round tip.

My "1M" tip is not really Wilton's 1M, it is some off brand that doesn't even have a number on it but it is just a big open star.

What do you want to use it for? That might help us give you more info.