Other than if you were to order them, where would you find chains?
Farmer’s yard sales, Farm estate auctions, Farm section in craigslist, Hell advertise in Craigslist you want some.
Hardware stores are usually a good place to look as well, lots of variety but prices may not always be the best.
Since you’re in the US, McMaster-Carr has a good selection as well.
Find a hardware store.
I found an industrial sling and chain place here in Nashville. Called them up and they had very reasonable prices. I also score some hyooge 3/4 chain for $5 / ft because it was “used”, as a safety screen for a rock crusher.
Not sure about what’s available in store and what’s order-only in most US hardware stores. In Poland, I ordered 10m of chain (about 55-65kg) and got it cut into 10 equal pieces. In addition to that I bought two lighter attachment chains, about 1.6 metres each. I bought 10 large carabiners for the heavy chains (D or oval would be ideal, but I could only get pear shaped), each one roughly the size of the chain links. For the attachment chains I bought 2 smaller carabiners. Doesn’t matter much what the shape is here.
Depending on your strength level you might want denser chains (I think the heaviest ones I was looking at were about 8-10 kg per metre, although obviously more expensive). If you’re tall you might want a longer attachment chain or 4 carabiners (so that you can make 2 loops on each chain instead of one).
I picked the length of each chain (just under 1m, taking into account the losses due to the links that got cut off probably about 90cm) based on the bar path on my main exercises, which is about 50-60cm. That leaves enough of the chain so that I start with 1-2 links off the ground and have 2-3 links left on top.
Two 5 foot 5/8 sections for $95 with free shipping (22 lbs each). Most hardware stores only carry up to 3/8 which is about 7.5 lbs per 5 feet.
Also makes really solid equipment, I can vouch for the sled and giant cambered bar.
Harbor Freight has 35lbs of 3/8" chain on sale right now for $17.