I2S to SPI or some other normal interface
This page on the TI website lists a dozen 24 bit ADCs interfacing with SPI. Speeds start at 15 sps.  

A thought on resolution:
24 bits or more! You do realize that 20 bit offers 1 ppm resolution, and that 1 LSB in 24 bit is 0.06 ppm of FS? Just looking at the circuit will give you several LSB of error, so to speak. For one thing, you'll need temperature control to within tenths of a degree to take advantage of the full measurement range.
If you really need more than 24 bits, TI has the ADS1281 which offers 31 bits.