... unpolymerised formaldehyde, H2C=O, not the solid, polymerised, paraformaldehyde or trioxane. It is gaseous, commonly sold as a solution in aqueous methanol in concs between 30% and 40% w/w. It can be obtained by heating paraformaldehyde with a little 50% H2SO4 IIRC. The gas produced would then be bubbled into cold DCM with MgSO4(anh.), this would be stirred and filtered to give anh. formaldehyde in DCM.
Both aqueous formaldehyde and paraformaldehyde can be obtained cheaply as a disinfectant for dairy farms, or probably any farms with livestock, and also as disinfectants for portable toilets. Another (expensive) source for paraformaldehyde is resorcinol glue, which as far as I can tell is a wood glue used mainly by boatmakers. It's a two part glue, one part being paraformaldehyde, the other is resorcinol, 1,3-dihydroxybenzene.
Edit: forgot, another good OTC source for trioxane, formaldehyde's trimer, is from army surplus stores as fuel tablets. They are generally packed in green foil "envelopes", inside brown carboard boxes, perhaps 6" by 3" by 1". These are not pure though, they contain some other freaky stuff, which sublimes, and if allowed to sublime in a closed container will form very long, slender crystals that are flexible enough to be tied in knots. They quickly disappear without trace if left in the open, and burn very easily, with an alcohol-like flame. But that is irrelevant.
Also, formaldehyde gas can be obtained by adding hexamine (a different type of fuel tablet, also very useful) to warm acid, such as HCl(aq), but this has the disadvantage that the formaldehyde produced will be very damp, and may be contaminated with HCl, if that acid is used. Doing it with nitric acid is just scary, and rather dangerous (nasty oxidation!), although perhaps dilute sulphuric would work...
