The chains that I have are from a lawn and garden tractor. They are fairly light duty but good enough for what I need. As DundeeBarnBuggy suggests, your local small tractor dealer should be able to find you some or have a look on your local buy and sell site ... people are always selling chains around here ... but this is Canada, so that make sense
You will need to ensure that you have enough clearance for chains or will need wheel spacers to create additional clearance.
I was lucky enough that my wheels are slightly offset (hub is not exactly centered), so if i reverse my wheels (with the valve stem pointing in), I can create enough clearance for the chains (I need about 1" extra). My wheels holes happen to be tapered on both sides (like tractor wheels), so that comes in handy too.