The USRP has a 64 MHz clock onboard which gives a 64 MS/s ADC and 128
MS/s DAC. To change that, you can use the external clock mode, or you
can replace the clock with another one. The pin-compatible family is
from CTS and contains many different frequencies. The 64 MHz clock is
part number CTX286LVCT-ND from Digikey. Make sure you get an LVCT
version.
Matt