I think you can use standard gas sensor that there are in eshops. if you want to work on sensors not using their, optical way can be suitable.

Gas standards in cylinders are not available at those low concentrations. You can get standards or even commercial mixtures as low as 2 ppm, so then you can perform a dilution step to the desired concentration. As formaldehyde is very reactive, dynamic standards (such as http://www.sciencedirect.com/science/journal/00399140/108) are preferred to static standards (gas cylinde rs). However these are pretty expensive to set up, so it all depends on what resources you htave and how accurate you need to be.

In the old days (1980s) I would use calibrated/certified "diffusion tubes" and dilution gas.