It seems the best deals (cheapest) are finding large diodes at surplus outlets. It is unlikely you will find a packaged bridge for the required current levels. Search with Google or have a look at Ebay.

You need a rectifier that will handle the current when the machine is in current limit (rod stuck to the workpiece). For my old Miller 225 I consider it to put out up to 200A on low range and 300A on high range. The diodes should have a minimum rating of 150V.

Some have paralleled diodes from old car alternators (about 4 sets) but if you put diodes in parallel you must ensure that the load is split somewhat evenly between them so one diode with a slightly different characteristic doesn't carry too much current and pop which then transfers the load to another which pops and so on.

I made a rectifier using 12 35A 600V bridge rectifiers with equal lengths (about 18") of number 18 (I think) wire feeding each connection on the bridge. These lengths of wire added about 0.04ohm resistance for balancing.

The best way though is to find 4 giant diodes to make life simpler.

International rectifier 150EBU02 150 amp 200 volt $7.23 each from Digikey. The 400 volt version is a couple of cents more. You would need four. Since each diode only conducts half the time in a bridge this should be a 300 amp bridge (if I'm wrong about rating somebody correct me please). You would also need heat sinks and such. International says this device is designed to go into welders.
Bill