So you pretty much want a spreader chain, but any type would likely work out.
There is roller chain, flat chain, & T-bar chain that would work for you.
Flat would work out as good as any for you, if you get sprockets to match. Or you can use roller too. Bar is a little odd, might not want to try to design your own with that type. (Note - I see my link below has T-bar links, so never mind....)
So you want only the chain, not the whole assembly, correct?
If you are making a 4 foot long spreader, you would need 2 pieces of 8 foot chain, plus, hum, 8 left hand & 8 right hand 'tang' links. You will make your own cross pieces out of angle or low-T iron?
You can get that chain, and the special links with tabs, from ag supply places, such as Tractor Supply, Valu-Bilt, and most stores with the word fleet, ag, or supply in them. Likely a good Napa store can also get them. There will be differewnt sized links. Heavier is better on a spreader.....
For pics, go to:
http://www.valu-bilt.com/
Click on chain, sprockets, & hubs to the left
Click on Steel Link Chain/ Attatchment Links
And you will see an assortment.
If you are flexable on what you need and can adapt the box size to the apron - do you live in an ag area? There are typically farm 'bone yards' or junk yards where they part out old ag tractors & combines. If you can find the right sized combine, they have a whole chain & cross link assembly both up in the feeder house (behind the header) which is called the feeder house; and also behind the cylinder, or the lower middle of the combine called the 'raddle'. If you find the right sized busted up old combine, you would have the whole chain & cross members all assembled for you, and can pick up fairly cheap. Then build the spreader to match the size of this assembly. Just a thought.
--->Paul