If you can't buy the parts, why do the calculation? Standard Values uses EIA standard resistance values in five tolerances to find the optimum solutions for Ohm’s Law problems (including power), voltage dividers, inverting op-amp gain, noninverting op-amp gain, as well as checking entered values against the EIA standard values.
All calculations are optimized to find the best value pair taking the tedium out of doing multiple calculations to find the best result. Doing the calculation manually (picking one value and solving for the other) only results in the best solution by chance.
Standard Values solves for which ever values are missing. For instance, in the voltage divider solver, entering voltages solves for resistance values while entering resistance values solves for the output voltage. When there is not enough information entered help is provided. If desired a nonstandard value can be entered and the entry will be honored.