This somewhat depends on what multimeter class you are dealing with. Rather surprising, the top-of-the-line bench multimeters (6.5 digit and up) don't rely on high-accuracy resistors at their input. Those fancy laser-trimmed resistors are rather for the ratios between different reference voltages and such in the actual multislope A/D converter.
What they need is a resistor that won't drift - both over time and temperature. They could very well be 5% tolerance parts, as long as they do not move away from that. Why? They can just calibrate any error out. If you know the error, in the divider, and you just apply the opposite transform to your measured data.
The reason they do this is because it is just impossible to get a resistor that is that accurate, and even if you did, just hooking it up to a circuit will blow that rating away. For illustration, let us consider a 0.001% resistor, which is the best I have ever seen on the market (and we are talking >100 EUR/USD per resistor here). That is still only 5.5 digits. It is far easier to just get a resistor that doesn't drift, and compensate for it after the fact.
Also, most of these high range multimeters actually have a much higher input impedance at low ranges. Of the top of my head, the Keithley 2000 has a rated input impedance of >10 GOhms for all but the two highest ranges (so up to 10 volts full scale). The range changes the gain of the amplifier, and doesn't use a input divider to scale. (ofcourse, at 100 V and 1000 V range this is not really possible since you won't find a JFET that can handle anywhere near that voltage. They use JFETs for the combination of input impedance and 1/f noise).
In lower-end multimeters (hand held ones) you might indeed find precision matched networks on ceramic carriers to do this input division. Alternatively, you can also use calibration, though that might not be desired as it is an extra processing step.
TLDR; the reason you can't really find them is because in the applications you want to use them, people don't need that accuracy - they use calibration instead.