Have you considered building your own??
We just built one that is 4 ft wide and 38" diameter (4' x 10' sheet of steel rolled) It weighs 600lbs dry and can hold 235gal of water which is another ~1900lbs. It does the job and we have less than $300 in it.
I just ran some #'s and at 6ft wide, in order to hold 500gal, it needs to be
46" diameter. Which would require a 12' sheet of steel, which is longer than standard and would add to the cost, as would the 6' wide, unless you can find an old 4' diameter section of pipe somewhere.