I looked into doing this a while back and for what heavy duty welding wire cost (without the cost of the clamps) I couldn't make them for twice what a decent set from the local auto-pro cost me.
A minimum of 200 amp capacity is needed if you're going to persist.
was out with some mates a while back, tried jump staring a patrol with normal jumpers and no go, when we hooked up old mates leads that were about 2 or 3 times thicker than normal ones she went straight away.
the guy who owns the cables reckons they make a huge difference.
Projecta do a large range of jumper cables and stuff. 6m and 1000 amp capacity would be heaps http://www.projecta.com.au/Products/BoosterCables/BoosterCables1.aspx#!prettyPhoto[iframes]/1/
But any electrical wholesaler should be able to do you some 50mm or 70mm squared cable that should be right, Just tell them its for a welder other wise they might try to refuse to sell it to you.
Ye I priced 500 amp leads in boc and it was like $12/m. And I don't know where to get the clamps either. I just don't wanna get the supper cheap auto leads, I wouldn't have much faith in them!
Just do a google search for jumper lead clamps, they come in a wide variety of sizes and configurations. Match the size to the use and you'll be fine.
Mine originally fell out of the stores at a power station, dunno what brand the cable is I haven't looked at them it in years and the clamps were found at a truck wreckers yard.
We made several sets of leads from the stash we got about 20 years ago mainly for jumping the tractor but one set which is stored in the shed because we no longer have a desperate need for such length and is too big to cart around is 7.0m long. It's easily done but may not be overly cost effective if you have to buy all the bits.
When I made my set, from old welding leads. I got the clamps from CTP on Melbourne . (Coburg truck parts) but any truck spares mob should have what you need