From another blender file, choose File > Link and browse to this file. Go in to "Node Tree" and select "Input IOR" to link.
Back in your original file, with the node editor open on a material that requires an IOR value (i.e. a Fresnel Node,) select Add > Group and choose "Input IOR".
You will see a large node with a number of common materials. Choose an appropriate material and connect that to your IOR input.
Some IOR values exist as a range. Those values are indicated by an asterisk (*) after the name. For those values only, the "Range" slider will allow you to vary between the minimum and maximum IOR value for that material.
The "Input IOR" node only exposes commonly used values. If you can't find what you're looking for there, go again to Add > Group and look at the other "IOR -" groups. They are arranged by material type, and have many more values to choose from.
The list is by no means exhaustive, but might save someone a bit of searching.
Tip: In Blender you can toggle visibility of unused inputs and outputs of a node with Ctrl-h.
So there are three new node groups you can link, as per the instructions above: "Colors - Metals", "Colors - Gemstones", and "Colors - Blackbody".
The gemstone colors are just approximate values from various sources, mostly Wikipedia.
The metal colors are calculated using refractiveindex.info, and have both facing color and edge tint values. For a good explaination of how to calculate your own, see here. For an even better explaination of how to use them, see Cynicat Pro's excellent set of video tutorials about PBR on YouTube.
The metal colors include a gamma slider. The calculated values are I guess based on an input light intensity of 1 and are not gamma corrected. The slider allows you to gamma correct all of the color values equivalently in one place. I find a value of 2.4 (that's two point four) works well, but the default remains set at 1.
The blackbody colors are intended as inputs to the color of an emission shader, the source for the various values, after some searching, was unsurprisingly Wikipedia. As with ior values, where the color temperature was reported as a range, the output name is suffixed with an asterisk and for those outputs only the range input allows you to vary between the lowest and highest value.
I've also used refractiveindex.info to verify the rather odd values of IOR for Gold and Silver, they both come out as less than 1, which would seem an impossibility since 1 is the IOR of a vacuum, but that's what they seem to be nonetheless, and it corresponds with various posted values for gold and silver around the web.