General ? if you cant find parts such as pressure hoses or return lines could you custom make them as long as they can hold the temp?
Are you talking about A/C systems or Hydraulic systems?
It's the hoses from the radiator to the resevoir.
Should not be a problem, parts stores carry a lot of universal hoses of different sizes and lengths, and smaller heater hoses, but I would recommend when using the smaller hoses try to get silicone hoses as they will stand the heat better.