www.dl5swb.de/html/mini_ring_core_calculator.htm
It really depends on, what current, power, what size, what accuracy?
yes it is quite possible. if you have a LC meter just buy a ferrite or iron toroid or rod and wrap the wire that can conduct your current around it and at each turn measure inductance until you get 200u. you can also wrap it around a temporary rod and make a air coil but it will be much bigger. if you don't have LC meter search google there are sites that give an approximate equivalent inductance for air coils with a known wire.
regards,