The HP/Agilent e3610a DC Power Supply requires this resistor for calibration, however I've searched the top 5-6 websites (digikey, mouser, etc) and can't find even a whiff of the part even existing. Any ideas?
You don't need it. Just use the 20A current range on your multimeter.
Get a 1% accurate unit and measure the exact resistance with an accurate meter or get several lower power parts and parallel/series connect them to reach the power rating.
Or, do what amspire says and just use a calibrated current meter rather than a precision load.
Note that the resistance will change as you increase the current due to heating of the resistor. This may be more than 0.1% depending on the temperature rise.
Why not 10 x 0.1% 1 ohm / 1W resistors in parallel?
Maybe they are easier to find (not sure, I've searched a little and found only expensive one, limited to 0.33 W).
regards
One thing about rolling your own: .1% of 0.1 ohm is 100 microohm. Obviously this is an order of magnitude less than contact resistance so you will need a 4-point kelvin connection. A commercial precision current sense resistor will have this done correctly. However, at this level spreading resistance and lead resistance are also significant. If you build a composite resistor from multiple lower power devices you will need to ensure you do so in a way that limits these sources of error. Of course, you can just measure the resistance with an accurate resistance meter, but if you have a sufficiently accurate ohm meter you can just take a 10W 1% resistor and measure that.