You cAn just buy bulk chain by the foot at Walmart,Lowes,hardware store,etc....just about any kind works for the side chains,"Passing link" or "Twist Link" makes better cross chains than regular links, but all types will work---you can get "monkey-Links" to assemble them at better auto or truck supply places..
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.