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The "Ergo Grip" is pretty much your HK91 type grip, it's the grip sold as the "HK91" grip in the Bushmaster catalog as well.
There are a few variants out there now where back when I purchased mine there was only one version which was hard plastic and had raised portions on it that were slightly more comfortable for right handed users but I still consider the grip to be ambidextrous. There's another version of the grip out there where it's perfectly smooth on both sides making it even more ambidextrous I guess, they also have versions with a rubbery texture on them.
Main complaint about these grips that I hear most people make is that of the hardness of the plastic, sometimes the things have been chipped or broke while others can't seem to ever get the grip screw tight enough without fear of stripping the threads of the receiver.
Now if you mean the HK PSG1 type grips, I don't have any experience with those. I've found one made by Sierra Precision Rifles that I like quite a bit more than that palm shelf idea used on the PSG1 grip.
Both versions of HK grips can be found all over the place though. Shopping for the best price though I have no clue, maybe try picking up a ShotgunNews or a GunList to look for ErgoGrip ads. Going right to Bushmaster or Brownell's might not be the cheapest route.