How can I make the lines on a buttercream cake, like the photo I attached? Maybe a decorating comb? If so, where can I get the comb that was used on this cake? I tried looking up combs on amazon & global sugar art, but there aren't many to choose from. Or did they use something else?

They may have used a brand new trowel found at the hardware store for tiling or the like?

Looks like some sort of comb to me...maybe check at an arts and crafts store?

It looks completely free hand to me (or just bad comb work), so it very well could have just been a wooden skewer or something that they dragged around the cake.

It you want nicer lines (and who wouldn't?!) find or make a cake comb - http://cupadeecakes.blogspot.com/2013/04/buttercream-comb-cake-and-comb-tutorial.html 

There are also plenty of options at the hardware store, or even a Wilton tool at your local craft store that is used for marking cakes (can't remember the name, but it was basically just a sharp stick that could move along a ruler haha)

AThat cake surprised quite a few people when it made the rounds on Facebook, considering the person who made it. Some of us figured the photographer or stylist must have mussed it up somehow. But who knows, maybe it looks exactly the way it was intended to. Maybe it's a new look that will catch on. The flowers are gorgeous, as always.

Wouldn't it have been funny if the person who made it started one of those what-do-you-think-happened-to-this-cake threads? lol I wonder if it would have been as pretty if it had been "perfect". 

AYou can always tape toothpicks, small dowel rods or what ever you want to use to a L shaped ruler and see if that works. The lines will be more perfect though LOL. Wilton also sells a tool with exchangeable tabs that give different effects like that. I sold mine in a yard sale because it was always slipping out of place and poorly made. That would actually explain the look of this cake Haha! Good luck :-)

Square-toothed combs will give the same effect - examples of icing combs