Mokuba (38th Street b/w Broadway and 7th Ave) is, without a doubt, the best source for ribbon. In fact, it is the only thing they sell! The first thing you'll notice when you walk in, is how well organized everything is. It is very clean with lots of room to move around the racks. They have tons of different widths, patterns, colors and finishes to pick from so it would be practically impossible for someone not to find what they are looking for. The only downsides to the store: the people who work there can come off a bit unpleasant. They will help you if you ask for it, but the last couple of times I went here, I didn't get much of a greeting or any offer of help. It can also be a little more expensive to shop here, considering the fact that all the ribbon is manufactured specially for the store in Japan. But if you can get past those two things, I would absolutely recommend coming here.
Daytona Trimmings (39th Street b/w 7th and 8th Ave) is also a place I would recommend As soon as you walk through the door, you see the entire right side wall covered in rolls of ribbon. Something this store does particularly well is embellished and patterned ribbon. There are some really great jacquard printed ribbons that always stop me on my way in or out the door. They have many widths and colors to choose from, and the prices are appropriate. The one problem I have while shopping here, is the organization of the ribbons. Everything is placed according to type, but a lot of times, the ribbons are coming off the roll, or the colors are all mixed together and it can be hard to focus and pick something out in the mess. Regardless of the messiness, the store has a great selection of ribbon and it is worth the extra time it may take to find what you are looking for.
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And, as always, I would try Pacific Trimming (38th Street b/w 7th and 8th Ave), Joyce (38th Street b/w Broadway and 6th Ave), and TrimArt (38th Street b/w 7th and 8th Ave). None of these three stores has much of a ribbon selection, but they are all worth a look.