IR LED Detector question
Transmitters
No, there are more than 850, 880, and 940nm transmitters: the IR spectrum ranges from 700nm to 1mm.  A distinct set of values is sold, typically in the 700 to 1400 nm range (IR-A), where 850, 880, and 940 are common values.  Here's a selection of the Mouser listing of IR emitters.  I've used the TSAL series of emitters from Vishay, although I'm not sure that they're in your current, brightness, or bandwidth specs.

The parameter you're interested in for transmitters is the "Relative Radiant Power vs. Wavelength", and for receivers, you want to know the "Relative Spectral Sensitivity vs. Wavelength".  For instance, the bell curve in Fig. 9 of the datasheet for the TSAL6100 shows that it has a relative intensity of 1 at 940nm, and outputs about 0.125 times this intensity at 890 nm.  That likely means that it's not bright enough to use with an 850 nm detector, and would be iffy at best with an 880nm detector.  

Receivers
On the plus side, receivers are usually more generous, for example the TSOP348 detector [picked at random] has a spectral sensitivity of better than 80% for all wavelengths between 850 and 1050nm.  Taos Inc. also makes some nice digital, analog, and frequency output detectors for many wavelengths; I've used them with good success before.  This will help you if you need to replace a sensor, especially if it's just used as an on/off digital sensor, for instance in a light curtain application, because 80% is pretty close to 100%.

However, that sort of receiver will only tell you about the quantity of light.  If you knew that your LEDs were the same brightness (you don't), then you might be able to infer a frequency (i.e. this one is 75% as bright as the 950nm, therefore it's about 820 or 1070nm).  You can also determine that an LED is on with just a digital camera, like the one in your cell phone.  

Color Sensors
An infrared camera could tell you the wavelength after compensating for temperature, but would not fit most budgets. (Note: These are awesome for determining all kinds of things - Night vision, temperature gradients, etc.)

What you need for that is an color sensor in the infrared range.  A color sensor will have multiple narrow-band and/or filtered detectors, so that you can determine the color of the light.  See Figure 1, Photodiode Spectral Responsivity" of the datasheet for the TAOS TCS3200D[pdf]  for an example (No, it's not going to be a pretty algorithm...). However, you'll notice that the visible light filters stop at about 750nm, and everything goes back to the same curve.  Finding a color sensor that works into the infrared range is left as an exercise to the reader, but this sort of IC is what you're looking for.   

An alternative to an IR color sensor (which may not exist) would be to use a broadband sensor with a set of infrared transmitting filters tuned to the region of the spectrum which you need.  A quick Google search turned up this page, you'll probably find something better.

Distributors:
As for distributors, I find that Mouser has a better selection and cheaper prices on optoelectronics than Digikey.