This works remarkably well:

http://sci-toys.com/scitoys/scitoys/light/cd_spectroscope/sp...

https://lavinia.as.arizona.edu/~mhammer/outreach/cdSpectrome...

(diffraction spectrometer from used CDs!), very easy to make, and it costs ~$0.

It has enough resolution to distinguish individual lines of fluorescent lamps (by using a narrow aperture). Highly recommended (very nice as an educational tool also, to impress folk with the hidden spectra of light around us ;) ).