Based on the type of their questions, I assume the OP does not actually need a high-precision current source, but just "any old constant current source that actually works, kind of".
@Arsh ahmad: Did you understand the explanations given so far?
- Any current source will keep adjusting its output voltage as the connected load resistance changes, to keep the current constant, so that U(t) / R(t) = I_target. That's what a constant current source does.
- But the current source can only adjust its input voltage downwards, so you need to supply an input voltage U_min which is high enough: U_min >= I_target * R_max.
- If your original voltage supply (battery pack, or whatever) does not supply at least the U_min voltage, a step-up (boost) converter is required between the supply and the constant current source.
Do you have a goal for the required precision? By how much can the actual current change? E.g. is 200 mA +- 1 mA acceptable, or what other tolerance do you have?