Two things you must look for when buying nutrients for hydroponics: 1) Must be water soluble 2) Must contain NPK + Secondary nutrients + trace micronutrients
A lot of "Soil" fertilizers only contain NPK and maybe some Mg, Ca, and S, but usually no micros because 99% of soil already contains them. So just make sure that you purchase a "complete" water soluble fertilizer. The other option is ordering your hydro nutes online.