All electronic hobbyists need to have some basics equipment in their workstation. The iron solder and its accessories are the main ones. I live in a small city and I faced many problems to find some kind of parts (cheap) like sponges. So I have found a good soldering sponge these days and I will show you.
Soldering sponges are vegetable, not a natural plant sponge like this, but a mixture of cellulose fibers that are industrially processed. Thus, it resists a considerable high temperatures (~100ºC) like cotton. To use it with your iron solder, as a normal one, keep it wet.
There are two ways to find it, I'll describe: supermarkets and drugstore, the prices are almost the same. For both, before first use, wash it in clean water. See next steps.
UPDATE: As our colleague pfred2 says, you can find it also from the 'dollar stores'. Just make sure it comprised just by natural fibers (cellulose) avoiding plastic compounds.
Obs.: English is not my first language, so if you find any misspelling problems, please let me know.
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Step 1: From Supermarket - Yellow Sponges
From supermarket, Fig. 1.:
3 sponges of 92mm x 70mm x 28mm, by the price of R$10 ($4), Fig. 2-4.
You will cut each one in four pieces like this, Fig. 5
To do that use a bread knife, Fig. 6.
You'll get 12 yellow sponges with 46mm x 70mm x 14mm, Fig. 7 ~R$1 ($0.5) each.
Actually the producer of this sponge is French: Spontex.
Step 2: From Drugstore - Pink Sponge, More Cute
From drugstore - 2 sponges of 70mm diameter x 10mm height ~ R$4 ($1.5).
This is not a yellow sponge because actually its pink, but it works nice :) The composition of those ones is the same.
If you would like to cut it, just proceed using the bread knife. I would rather compress (shrink) it to fits the holder size.