Bessey is the gold standard for metal working clamps these days. Take a look at this page:
http://besseytools.com/en/subcatego...600030023&ASIMOID_SC=0000000100030a6400020023
Note that truly heavy duty C clamps are actually called 'bridge clamps' by other companies. They are not cheap.
That said, are you absolutely sure you want a C clamp? I hate the things, as they always twist my work as I tighten them the very last bit. Most people I know who fab the screw types of these rather than 'C' clamps for the added capacity, and I actually switched over to the lever types (with red handles) shown here so they also don't ever twist the work:
http://www.besseytools.com/en/subca...300030023&ASIMOID_MC=000000000001f07600030023
and
http://www.besseytools.com/en/subca...500020023&ASIMOID_MC=000000000001f07600030023
I also keep a bunch of these around (vise grip 310s, 310r, 318s, 318r -- the 310r are probably my favorite among these). They are not as robust as others, but you can use them one handed with some practice:
Likewise, smaller Kant Twist (1", 2", 4", 6") have replaced my smaller 'C' clamps unless I'm doing something that will destroy a clamp. They are fantastic, and you can find them up to pretty large sizes (12"+):
If you don't want to pay Kant Twist prices, then the Strong Hand Tool shark clamps (SC50 for example) are very similar and have 1000lb of clamping force -- These would be my go-to if I wanted to buy C clamp replacements that were strong and still cheap:
http://www.trick-tools.com/Shark_Cl...SSFDgUliCCGiRXa1fi1smk4Tfmt8AvAqesaAjVe8P8HAQ
And if you really want something monstrously heavy duty, then check out Carver rack clamps. They're rare stateside, but industrial welders in the UK love them:
http://www.carver.co.uk/prod_rack.htm