I'm not sure you need to know the internal resistance of the analog pin on the Arduino itself as it's not a continuous connection to the input. What's there is a small capacitor that is connected and charged for a short time, then the pin is disconnected from your source and the capacitor is sampled to arrive at the analog reading. In the ATmega328 datasheet
The ADC is optimized for analog signals with an output impedance of approximately 10 k Ohms or less. If such a source is used, the sampling time will be negligible. If a source with higher impedance is used, the sampling time will depend on how long time the source needs to charge the S/H capacitor, with can vary widely.
Your instrument specifies a load of 10k Ohms or more, so if you used a resistor divider with two 10k or greater resistors it would be satisfied. To reduce the input impedance to something the Arduino analog input will prefer, you could simply include a 0.1uF capacitor from that analog input pin to GND so when the sample is taken, it's from that already stored voltage in the cap.
There's a discussion of something similar (though a more extreme resistor divider is discussed in this thread
where other options are proposed also. From that thread you'll see it's quiite possible if your sample frequency is low enough you won't even need that cap.