You can make these pads; even wash and reuse them to save you money on buying the disposable ones. This is a guide about making replacement pads for a Swiffer.
Read and rate the best solutions below by giving them a "thumbs up".
I recently made a Swiffer Cozy for my Swiffer to replace the dry Swiffer cloths I buy. I think it qualifies as the ugliest Swiffer Cozy ever, but it used up leftover yarn I had and, it is very functional.
I just measured the size of the bottom of the swiffer, and crocheted enough to cover that space, then added the top sides to go over the edges; making one side smaller than the other, so as to be able to slip it on easily. While it is ugly, it works great!
Best of all, when I am done, I shake off the dirt, toss it into the wash, and I do not have to keep buying the swiffer cloths and tossing them into the trash.
I hope to also make a mopping pad for my Clorox ReadyMop; using either cotton yarn or by sewing one with different materials.
By hoptownracer1 from USA
I use old T-shirts and sweatshirts to make replacement pads for my Swiffer Sweeper. I just cut rectangles large enough to fit around the Sweeper. Make sure you have enough left to poke into the holes that hold it in place. I use the rest of the shirts, sleeves and all, as dust rags. They can be washed or discarded.
By Luci M. 
I bought a basic 'swiffer', not the battery operated kind. Instead of buying the wet sheets, I use thin wash cloths. I wet them with detergent water, and squeeze it out and they fit on perfectly into the little holes. It is fast and it cleans really well!
By Bubear from Pittsburgh, PA
Using a wet Swiffer mop can get expensive with the wet replacement towels that cost about $4.00 for a dozen. It adds up really fast! To combat that, I decided to try using some old thin bath towels cut to size.
I dampen them them with a cleaning solution of about 2 Tbsp. Murphy's oil soap to 2 quarts water, place them on the mop as I would the Swiffer cloths, and clean my laminate and tile floors.
Cloths can be washed and reused over and over. I save the cleaning solution in a container and it lasts a really long time. The cost is pennies compared to what I was spending! (Flannel cloth can work great as well.)
By Debbie from Fisher, IL
I love Swiffer type cloths. I found when I ran out one time that you can use cut up rags (esp. flannel) the same size as one of the duster sheets and attach to the mop the same way. Voila! It works great and you can just throw it in the wash if you want to reuse. You can also lightly spray the cloth or dampen it and wring out to do a little spot cleaning. This is a great way to reuse pillowcases that are very worn, or any old thing.
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Here are questions related to Making Replacement Pads for a Swiffer.
I would like to know how to knit or crochet a reusable swiffer pad.
By TT from Douglas, WY
By Harlean from Arkansas 12/07/2011
I have tried everything from old socks to used dryer sheets, but for the past year I have been using a Microfiber cloth. I buy mine at Sam's club, and cut them in half. One half is just right for the Swiffer and to tuck it in the slots. They don't fray, so you don't have to set the cut edges. Each cloth makes 2 swiffer pads.
When I finish sweeping with them, I use one of those scratcher pads that is like a covered sponge, and brush the lint, dog hair, etc off the pad. When they get a little grimy looking, I just throw them in the washing machine with the throw rugs.
Harlean from Arkansas
I think Swiffer cloths are too expensive. I live in an old house and have hardwood floors throughout in addition to two dogs. I am leery of some suggestions for cleaning fluid, washcloths, and the dogs on the wood floors. Occasionally I stumble on dry floor cloths at a dollar store, but not often. Any other ideas are greatly appreciated.
By UDN from Chicago, IL
By ItalianPear 01/27/2011
Reusable "Swiffer" pads are very simple, purchase large microfiber cloths and use them on a Swiffer or Swiffer type mop. I have used these to clean my hardwood floors, and I have two cats that shed all the time. If I want to mop the floor, I simply spray my favorite wood floor cleaner (I use Bona) and then wipe over with the cloth.
The only issue is that you have some very grimy laundry to do to reuse the cloths.
My Swifter wet mop has no Velcro to stick the pads on. They must have fallen off from too much solution. Any ideas of how I can replace them? Should I just use regular stick on Velcro from JoAnn's?
By catash 
How do I fasten homemade Swiffer wetjet pads to the mop head?