Yard sale treasures they are. A buck a pair usually. They may not be the correct width for your tire (my 2 sets aren't), but as T-H says, Lowes etc has the basic chain material. The 'special' links are on the yard sale chains. All you need is a vice, hack saw and vice grips to change one into the other.
Oh. Also need some patience. You may need a couple bandages, too, because they can get honorary sometimes, fighting back. Don't rush it. Take your time. Do it right.
My 18" tires will need about 31 ft of new cross links, and $1.25 per ft. at Lowes, well, it's cheaper than buying ones that should fit.
My local NAPA wouldn't talk tire chains.