Flat vs. American Curved vs European Curved? Huh? Yes, all marketing gimmicks to make you confused! For the last 5 years, 1st-line has been moving in the direction of only carrying flat tampers. We do have some curved one with attractive prices, but the bottomline is that we have found no differences in the end result in the cup as long as one knows how to properly make espresso.
We do have calibrated tampers as well as adjustable pressure tampers.
More importantly, acquire a tamping mat or station to preserve the beauty of your counter-top and to also get a level tamp over the grinds in your portafilter!