I'm a relapsed bullet caster. I cast for several years in the 80's but gave it up because of work, kids, house, etc. and just started buying my bullets. Back then I'd scrounge for metal but it was a lot easier to find. I worked in the printing industry and it wasn't hard to find linotype.
Fast forward 45 years I'm retired and I took it up again. Commercial bullet casters go out of business with surprising regularity. I got tired of that, and the fact that more than a few dropped their offerings for my beloved .41 Magnum. Not to mention periodic shortages. I thought it best to take it up again so at least I would be ready if there were more supply constraints.
I found scrounging and processing scrap difficult, but not impossible. Finding and processing scrap is as much work as casting bullets. So I decided to buy it. I found I could buy it for $3 a pound. Commercially cast bullets have climbed over $5 a pound (+freight) if you do the math. It depends on weight, with the smaller bullets actually more per pound than the big ones (makes sense if you think about it.).
The $3 I pay saves me 40%+ vs commercial bullets. I've mapped out my usage pretty well and the 5,200 cast bullets I shoot a year runs me $366 with the purchased metal. At my age I have to ask myself if all the scrounging and processing is worth saving a few dollars. If I bought my bullets I'd pay $637. The savings is a bonus, I didn't take it back up with cost as the primary focus.
Especially starting out, I'd recommend you buy your metal at first so you can concentrate on learning to cast. You'll be eliminating having any alloy problems to deal with. Once you have it down you can start keeping an eye out for scrap. Powder coating (PC) instead of traditional lubrication is fantastic, and requires little investment.