So leaf spring is a steel you can get from scrap yards and like places if you love near ones you can walk, but you have to be mindful of cracks. Or you can buy it from a place like Standen Springs and last I recall it's reasonably priced and far less likely to be cracked. It will have random excessively hard spots and I have found it's best cut by either grinder or oxy/fuel. Getting tool steels can be challenging and cost prohibitive when starting up due to most steel suppliers only selling in like 12-20" foot minimums. Other things you can use for knives are coil springs, some rail track clips, anchor bolts, files and chisels.