Parts store springs are more than ok as long as it is for the correct application. You can go stock replacement or get one of the variable rate "cargo springs" that several vendors advertise.
Moog, Duralast, etc.. all make a great replacement spring for our cars, and they cost no where near what you paid for your shocks.
Stock replacements are Moog part #5379, and cost about $45
Cargo Springs are CC627 and cost about $55.
Make sure you replace the insulators as well if yours are original, part #K6203, about $2 each
Napa, Advance Auto, and other parts chains should be able to get any of the above items for you.
Plenty of street AND race cars have done fine with either spring.
Billy
Montgomery Village, MD