Craigslist, outer duals only.
there's probably 500 different chains for sale at any given time in the Seattle area. Check it out. Other wise, Amazon, some other online place or Les Schwabs.
Good peace of mind here IMO. Never had chains in 20+ years of winter driving/towing til we moved here. Still wouldn't except we get back on forest roads with a large trai,er in the winter, snowmobiling and they are necessary in some places.
If I was staying on road I wouldn't have any.
Another consideration, if you need them for glare ice on the road, pick up at least 1 set of cheap cable chains for the trailer.