If I remember correctly, Amazon has some awesome prices on cast iron, usually around Black Friday or Christmas. Start browsing now and add the stuff to your wish list. Eventually, you probably see a price drop on some of it.
Yard sales, flea markets, thrift stores, good will antique shops etc. Problem with on line is cast iron is heavy and expensive to shift scoring a bargain dont do no good if ya hafta tack on shipping not only that prowling ar0ound looking for ti is fun.
Like Cappy said, yard sales. I like the older stuff though. And not to mention the pieces I would love would be triple digits new most likely. So I hope I can get them at yard sales, although I have yet to find any really big pieces.
Flea markets can be good. Depends on who is selling. It can get pricey.
Amazon has had some decent deals on new Lodge products. I hear Lodge even has an outlet store but I've never seen one.
We bought our cast iron pan at a Bed, Bath and Beyond store. They were having a sale so I scooped up a frying pan and a large grill to put on the stove. I'd never buy it full price because it's expensive.
The Lodge cast iron store is a good place to find stuff as well as Walmart. If you have an Old Time Pottery store around they sell cast iron pans and accessories as well. I just bought mine from the local Lodge factory store and I got a 12" skillet and a 12" grill for $30 total because of the sale they had going on at the time.
I've seen some decent prices for cast iron pieces on eBay, and also on Amazon. I agree that if you put the items you're interested in on your list, they will notify you when they go on sale. Walmart is the same, they'll email you when something you've shown interest in goes on sale.
I also agree that garage sales are good places, as well as your local Facebook sales groups. There are groups where you can post ISO (In Search Of) listings, and if someone's looking to sell those items, they'll let you know. Another good place could be local auctions and estate sales. You can find local estate sales here. https://www.estatesales.net/