Unfortunately, Home Depot and other box hardware stores only sell mild "structural" or "weld" steel barstock, which is not hardenable.
Buying some 5160 or similar bar stock from a knife supply or steel supply company is the only way to go if life or limb are at stake on the finished product. but then again, I wouldn't advise doing your own amature forge work if that is the case.
Your best bet for free or cheap spring is going to a spring manufacturer or junkyard and asking for scrap leafspring. You may luck into one that is already in the rough shape you need, but it can also be forged as needed.
If you forge it remember to normalize and then aneal at least once before moving on to final heat treating.
Heat treating scrap steel will depend on what alloy the original spring was made of, (5160 in old trucks, but any number of alloy spring steels these days) but in very general terms it requires heating to cherry red, quench in oil and then temper between 500 - 700 deg F. Lower temperature end gives more spring but may crack under heavy loads, higher end will give more flexability so it whont be as prone to cracking under a heavy load. Either way it should return to shape when the load is removed unless overstressed.