They exist, however the PB1400c is a pre-cardbus machine, so they arent compatible, or rather, I havent found one. (The other issue you may face is that theyll need OS 8/ OS 9 to be able to use the USB ports, or at least, Ive never seen OS7.6 use USB).
PCMCIA cards are 16bit and USB cards require the 32bit Cardbus.
Further, the 1400 series uses the old Nubus architecture and would probably be incompatible in some way even if a 16bit USB card existed (which they don't).