I suspect you do not need to use 670nm exactly, however I suspect you need to be above 650nm. Note that many red LEDs are at a lower wavelength (~630nm). So you need to search for deep red or by wavelength.

You can use a OSA, but yes they are expensive. Spectrometers are typically cheaper, but still expensive (you could go to a university optics group and ask them if they could measure it for you). That said if you buy from a reputable source you should get the right wavelength.

If you are trying this out be careful with the brightness.