Looks like these might work.
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200304461_200304461
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200304463_200304463
The company makes 7' ones as well
http://fivestarmfginc.com/712-arched-aluminum-ramps.htm
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200304461_200304461
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200304463_200304463
The company makes 7' ones as well
http://fivestarmfginc.com/712-arched-aluminum-ramps.htm