DSE definitelly used to have some M3 and M4 cheese-head nylon bolts, I bought a bag of them years ago, it contained about 8 or 10 each of 12mm and 25mm long screws, and about ~20 nylon washers and nuts.
They might not have them now though as DSE is getting out of all that 'fiddly' stuff(components, kits etc) and concentrating on TV's, DVD's, PC's etc, so you may have to try a few stores for remaining stock.
Oh and have a look for it yourself, if you ask one of the monkeys they will just say they don't have it, even if they do.(They're more interested in sales of laptops etc)
The Jaycar screws are nylon but they are clear and look more like acrylic... dunno tho, never tried them but they look inferior to the DSE ones. To see them type 'Nylon fasteners' into the search box at
www.Jaycar.com.au
Smallparts are awesome, though a little pricey on some stuff.
EDIT: Have just checked an old printed catalogue, the DSE packs are as I remember, 10 each of 12 and 25mm, 20 washers and 20 nuts.
Cat# for the M3 pack is
H1110(link to DSE), for the M4 pack its
H1130 and are about $10 each. Click the 'Availability' button for a list of stores that
should have stock...
Disclaimer: Jaycar employee, tho not phishing for sales. Unless you want to drive to WA