#3 zippers are the all-purpose zippers you can buy at any craft store, and they're what most POP ITH zipper bag designs use. If the design doesn't specify #5, then it is sized for a #3. #3 zippers have teeth that are 3mm wide, and almost always have tape that is 1" wide. #5 zippers have teeth that are 5mm wide, and the tape is around 1.25" wide. #4.5 zippers have teeth that are 4.5mm wide, and the tape is around 1.25" wide, and they usually have a longer "purse pull."
If you want the fun, metallic-look zipper, make sure the teeth are polyester or nylon. You do not want to use real metal or molded plastic in an ITH zipper bag.
What's the point of having two separate designs? What is the difference between the #3 and #5 designs? Nothing really when you look at them completed (which is how you know the design is done well!), but the zipper size is an integral part of the design. Jen thought it would be more simple to adjust the design for the #5 zipper than it turned out to be when she started with the Trendsetter. It's more than just scooting a line or two over. The #5 gets tested and tweaked in each size just like the #3 does before the design gets released.
Can I just "hack" the design for whatever size zipper? When people were hacking the standard #3 design using a #5 zipper, the lining was getting caught in the zipper teeth, and the embroidery machine foot would sometimes hit the teeth and throw the design off. There wasn't enough zipper tape outside of the zipper teeth for the zipper to work correctly and the embroidery foot to have enough room to maneuver. Conversely, if you're putting a more narrow #3 in a #5 design, the tape won't get caught by the stitching/not enough tape in the seam allowance to be secure or there will be too much tape showing which isn't as attractive.
What about invisible zippers? They usually have more narrow tape than #3 zippers, so the edges might not get caught in the tack down stitching. The zipper teeth are on the underside of the zipper, which means it doesn’t lay flat against the stabilizer and can be easily scooter out of place by your machine foot. The zipper pull is small, and some people find it fussy to grab and open. (Though you may be able to switch out the zipper dangle with a homemade bead pull or charm pull!)
Where can I get zippers? You will probably be able to find the best prices online. POP shops on Wawak for their #3 Nylon Zippers, especially during their Buy 2 Get 1 Free sales. (Price comparison as of today—a 14” Coats & Clark #3 zipper at JoAnn is $3.49 and a 14” #3 YKK zipper is $0.37 at Wawak during a B2G1 sale, making them less than 25 cents each!) ZipIt on Etsy is another great source for zippers, both #3 and #4.5 purse pull styles--and there's a 10% off code in our Facebook group! They're currently open limited hours (check their Zip It Facebook page for announcements) during the health emergency. For #5 zippers, try MyHandmadeSpace's website or their Etsy shop.
Choose the design option sized for the zippers you would prefer to use! We have these listed as separate designs to hopefully minimize confusion.