PSU for Ampacity test of connection
My quick and dirty options follow:


Use a step down transformer to generate a lower working voltage. i.e. something like 5VAC from 120V. Rectify and use bulk capacitance to allow a large and long enough current pulse for sake of testing.
If you require longer testing i.e. seconds or minutes, then I would suggest using common LiPo or Lead Acid batteries in parallel, and an approximate water load (water mixed with baking soda or salt make it conductive.) A 5Gal bucket and two copper pipes in it can handle thousands of watts of power for quite a while.
Finally if neither of the above options sound acceptable, then you need to start looking into much more expensive supply options. You could continue doing what you are doing, but add balancing resistors (~0.1 Ohm) in series with each supply. Consider investing in cheap Ebay style SMPS.