Must have at least 750 lb capacity.
I bought these as I already had planks laying around for my roof jacks.
Rated at 700ea.
http://www.lowes.com/lowes/lkn?action=productDetail&productId=21705-29318-0700900&lpage=none
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=44649
these are on sale for $59 or something and grab the harbor freight ad and most of the time you can find a 15% off one item coupon.