Did you try over the phone or online to book the Kanazawa to Takayama bus journey? If you are using a bus pass or Takayama Hokuriku Pass, you must call. If you are just buying single tickets, you can use the bus booking site. See link below:
If Kanazawa to Takayama is indeed full, and you have a Takayama Hokuriku Pass or a JR Pass, take Shinkansen to Shin-Takaoka and catch the bus there. Buses start from central Takaoka but there are often seats left at Shin-Takaoka. This bus does not take seat reservations. THP would cover the bus ride to Takayama as well. JR Pass does not.
If you don't have one of the rail passes above, then take the Ishikawa Railway, which becomes Ainokaze Toyama Railway (same train, no transfer) in the Tomari direction. Disembark at Takaoka (without the "Shin") where you can catch the bus.
Bus timetable for your reference: https://www.japan-guide.com/bus/gokayama.html