I do utility work and run into a lot of square head hardware. I'm looking 4-8 point impact sockets for a 18v impact driver for 1/4" lag bolts with 5/16" or 3/8" heads.
Jeremy
here's 1/2" drive - 8 point snap on 1/4", 5/16", 1/2" 8 point impact sockets for $20..
http://pages.ebay.com/link/?nav=item.view&alt=web&id=231578611674
if your gonna use an impact driver, and will use these alot - id suggest hunting down some 1/4" hex to 1/2" square drive adapters with a pin lock (if you buy those ebay sockets) . this way you can really tug on the them and there is no danger of the socket or adapter falling out of the impact driver, or falling down from heights.
i recently started using my 12v max dewalt impact driver at work, and found a listing with a guy selling 5 apex 1/4" hex to 1/4" square drive adapters with a pin lock for like $10. then i bought a cheap $20 sae , 1/4" deep impact socket set from amazon. so now i can use the 12v max impact driver on bug nuts / grounding hardware and stuff.. i dont see many square heads in that size, and if i do i usually just hammer them in, or use a pry bar to rip them out..
dont know your particular situation, but it might be easier and cheaper to just buy your own hex head lags if you are installing new stuff. this way you can buy regular hex head impact nut drivers.