I am looking for the absorption spectrum of THT. What is the best way to find these types of exotic material's spectral characteristics?
A good source of spectroscopic data is the NIST Chemistry WebBook. It compiles all possible information about polyatomic molecules including their UV/Vis and IR spectra. You will find references to corresponding papers and, as I see, the new version includes a Java applet that plots the UV spectrum. Here is a direct link to THT spectrum.