Are you having problems finding materials for face masks (not medical-grade)? Empress Mills in Lancashire is an online haberdasher which can supply elastic, various types of tapes and also fabrics which could be used for masks (thanks for the info, Cathy!). They will cut off a length of eg elastic specified by you (above a minimum length), and send it with p&p little more than the cost of the postage.
They do not have the NWPP material which some re-usable shopping bags are made from, recommended in Clara's design - it sounds as if this is the best material, if you are not making a medical-grade mask. If you are using cotton, at least one of the layers of the mask needs to be a tightly-woven cotton, such as curtain lining or perhaps percale bed sheeting (or some other high-quality sheet material). Old curtain lining and sheets could be used, or you can buy curtain lining from Empress Mills cheaply. Their all-cotton sateen lining is £3.80 a metre ('Solprufe'), 54 inches wide (mixed metrics!). Or Poly/Cotton lining is £3.90 a metre by 54 inches wide, £4.20 by 108inches wide. Curtain lining is designed to keep light out and warmth in, so is likely to be more effective than a lot of other cottons - particularly the all-cotton version, though this is also rather thick and hence less breathable (also important!).
If at least one inner layer is a dense cotton, then the outer layer could be some fun dress fabric. And increased filtration can be supplied by inserting a sheet of kitchen towel inside the mask - or perhaps half a sheet - and throwing it away after use. According to the designer of the Hong Kong face mask, filtration for his 2-layer design is increased as follows (details here, including breathability):
Filtration
Efficiency
Mask only (type of cotton unspecified) 8%
with sheet of facial tissue 45%
OR with sheet of kitchen towel 73%
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Here are some ideas for possible facemask fabrics from the Empress Mills blog - apparently they have had lots of requests. They say that Vilene interfacing is popular to use as a filter inside the mask - I wonder how breathable it is?
They also have suggestions for fabrics to make scrubs out of.