M Wave Design has a full line of coaxial and waveguide isolators for cryogenic applications.
The 3750HPC333 is a 375 - 500 MHz coaxial circulator rated at 2Kw average and 10Kw peak.
The 187IR1203 is a 'C' band isolator rated at 320 Kw peak and a 320 Kw average designed to protect high power tube amplifiers.