Are there any commercially available crystals in the sub 32 kHz range?
Some manufacturers refer to 100kHz ~ 1.8432MHz as "very low frequency". Below 100kHz you'll find only a few specific values like 77.5kHz for (DCF77 receivers) and of course the 32.768kHz watch crystal. You won't find standard crystals below 10kHz.
Nothing lost, by the way. They won't help you meeting your low power requirements, anyway. A crystal needs a certain power to maintain its oscillation, and if you look at the datasheets you'll often see drive levels in the order of 1mW.  

The lowest power microcontroller application using a crystal I designed was an MSP430 running at 32.768kHz, and that used 3\$\mu\$A, a thousand times your requirements.
This oscillator still uses 300nA, that's 0.3\$\mu\$A.